1
Introduction
Iran and its neighbouring areas are considered as a complex puzzle, in which
continental fragments of various origins were assembled and are now separated by
discontinuous ophiolitic belts within the Alpine–Himalayan orogenic system. The
formation and evolution of such a large orogenic system have been controlled by the
opening and closure of the Tethyan Oceans (the Paleotethys in the north and the
Neotethys in the south) (Stampfli, 2000; Stampfli and Kozur, 2007). Metamorphic
rocks, which are scarcely exposed in different units (Huckriede et al., 1962; Stöcklin,
1968, 1974, 1977; Nabavi, 1976; Berberian, 1976a, b), were interpreted as Late
Precambrian basement (Stöcklin, 1968, 1974, 1977; Haghipour, 1974, 1977). Similar
to other high-grade metamorphic rocks in Central Iran, the Shotur Kuh Complex has
been assumed to represent a Pre-Cambrian metamorphic basement. New geological
mapping in the northeastern part of the Central Iranian Zone (Rahmati Ilkhchi et al.,
2002) showed that the basement rocks are covered by Triassic sequence and that there
are no data to confirm a Precambrian age for this complex.
This thesis focuses on tectonics and on the magmatic, and metamorphic petrology
of the Shotur Kuh complex. In order to understand and seek mechanisms of
metamorphism and deformation in relation to geotectonic processes during geological
history, a brief summary of the regional geology and age dating from different
basement units in Iran is presented. The thesis consists of four chapters. The first
chapter is a compilation of available data from the literature about metamorphism and
geochronology data of metamorphic rocks and their protoliths. Chapters 2 and 3
include results of my research work that are accepted for publication and submitted
for publication, respectively. In general, these two chapters provide an
interdisciplinary approach to the metamorphism and tectono-metamorphic evolution
of the Shotur Kuh complex and emphasize the related links between geochronology,
metamorphic petrology, micro-structural and macro-structural analyses, and tectonics.
The last chapter (4) is a summary of the achieved results with their possible
interpretation and some future directions of research in basement units of Iran.
2
CHAPTER 1
Summary of Geology and Metamorphism in Iran
1.1. Geology and Tectonics of Iran
Iran represents a mosaic of continental blocks separated from each other by fold-
and-thrust belts formed during the opening closure of the Tethyan oceanic basins
(Gansser et al., 1981; Stampfli, 2000; Stampfli and Kozur, 2007). The Paleotethys
Ocean separated the Variscan domain from the Gondwana-derived Cimmerian blocks.
This ocean closed in the Late Triassic by northward subduction under the southern
Laurasian (Turan) margin. Rifting in the Late Palaeozoic to Early Mesozoic resulted
in growth of the Neotethys (Berberian and King, 1981; Talbot and Alavi, 1996;
Stampfli and Borel, 2002). Based on the presence of basement units and fold-thrust
belts, some with ophiolite, three major structural units can be distinguished in Iran
(e.g., Berberian and King, 1981). They include the Zagros fold and thrust belt
(southern unit), the central unit (Cimmerian block) and the northern unit (Fig. 1).
Further subdivision of these units is based on structural style, the age and nature of
magmatism, and the age and intensity of deformation and metamorphism.
Fig.1. Simplified structural unit of Iran with the Arabian Plate in the South and the Turan Plate in the
North (after Berberian and King, 1981).
3
1.1.1 The Zagros Fold and Thrust Belt (southern unit)
The Zagros fold and thrust belt with Precambrian basement and platform–type
sediments developed during the Palaeozoic. It is subdivided into three subzones
(subunits): the Zagros fold belt, the Zagros thrust zone, and the Makran, Zabol-Baluch
zone, including the Eastern Iranian Ranges.
Fig. 2. Geological sketch map showing the main Iranian geological zones (after Berberian, et al.,
1981).
1.1.1.1. The Zagros fold belt
The Zagros belt (Falcon, 1969) occurs in the southwest of Iran and bounds the
northern margin of the stable platform of Arabia (Fig. 2). It is an approximately 200–
300-km wide zone with a thick sedimentary sequence of Precambrian to Pliocene age.
The Zagros folded belt is characterized by NW-SE-trending, doubly plunging,
parallel anticlines and synclines, which are generally formed by a flexural slip
4
mechanism (Colman-Sadd, 1978). The basal evaporate-dolomite in the basis
sedimentary sequences has played an important role in controlling the style of
deformation and detachment mechanism (Kent, 1958; Falcon, 1974; Talbot and
Jarvis, 1984).The evaporates also cut throughout the overlying sedimentary rocks and
appear on the surface as salt diapirs. Within these diapirs, numerous ―exotic‖ blocks
of various rock types are present, which contain fragments of both the overlying rocks
and the metamorphic (schist and gneiss) blocks of the underlying basement. This
zone, formed in the late Cenozoic, is still tectonically active with shortening and
thickening due to the collision of the Arabian Peninsula with Central Iran (Berberian,
1995). The precise timing of the Arabia–Eurasia collision recorded in the Zagros
Mountains, with estimates ranging from Late Cretaceous to late Miocene time,
remains controversial (Dewey et al., 1973; Hempton, 1987; Alavi, 1994; McQuarrie
et al., 2003; Mohajjel et al., 2003; Alavi, 2004).
1.1.1.2. The Zagros Thrust zone
This is a narrow zone that occurs along the northern border of the Zagros fold
belt. Most of the rocks present here are deep-sea sediments of the Zagros Mesozoic
and early Tertiary basin. The Mesozoic and Paleozoic rock sequences are thrust
southwest in several schuppen-like slices on younger Mesozoic and Tertiary rocks of
the folded belt. Thrusting of Central Iranian substance above the Zagros was followed
by dextral wrench faulting along a Zagros fault zone (Ricou, 1976; Ricou et al.,
1977). The wrench faulting is still going on.
1.1.1.3. The Makran, Zabol–Baluch Zone and the Eastern Iranian
Ranges
The Makran and Zabol–Baluch zone occurs south of the Lut block (Fig. 1). It is a
belt of post–Cretaceous flysch–molasse in Iran that continues to the Pakistan
Baluchistan Range Makran, Zabol–Baluch zone, where it consists of Eocene to
Miocene turbidites and Pliocene to recent silts, sandstones, and conglomerates
(Falcon, 1974). The Makran Range corresponds to Cenozoic accreting margin,
bounding the northern, oceanic part of the Arabian–African plate (Gulf of Oman). The
Eastern Iranian Ranges are in a north-south direction and occur east of the Lut block.
5
1.1.2. The Central Units (Cimmerian Block)
These units are assumed to represent marginal fragments detached from
Gondwana and attached to Eurasia during the Mesozoic period and finally rejoined to
the Gondwanic Afro–Arabian plate in the Late Cretaceous. Their crystalline basement
consolidated during the Precambrian (Pan–African) orogeny, and it is covered by Late
Proterozoic to Paleozoic platform–type sediments. Alpine tectonics occurred during
the Cimmerian phase. The central unit includes Sanandaj-Sirjan, Urumiyeh-Dokhtar
Magmatic Arc, parts of Central Iran, Alborz, and Central and Southern Afghanistan.
Here, only tectonic units in Iran are discussed.
1.1.2.1. The Sanandaj Sirjan metamorphic zone
The Sanandaj Sirjan zone is a continuation of the Taurus orogenic belt in Turkey
and reaches length of about 1500 km in the territory of Iran. It is often considered a
hinterland that has been folded and metamorphosed during the Early Cimmerian
orogeny (Upper Triassic), tectonized again in the Upper Cretaceous, and finally
deformed with the Zagros Main Thrust. The Sanandaj Sirjan zone is subdivided into
an outer belt of imbricate thrust slices that includes the Zagros suture and an inner belt
of mainly Mesozoic metamorphic rocks. This zone is characterized by the presence of
metamorphic rocks with abundant deformed and undeformed plutons. The rocks of
this zone are mostly of Mesozoic age, but Paleozoic rocks are also present, and they
are common in the southeast part (Berberian, 1977).
1.1.2.2. The Urumiyeh Dokhtar (Urumiyeh Bazman) Magmatic Arc
The Urumiyeh Dokhtar Magmatic Arc (or ―Urmia-Dokhtar zone‖ of Schröder,
1944) runs parallel to the Sanandaj Sirjan zone along its northeast side. It contains
intrusive and extrusive rocks of Eocene–Quaternary ages that form about a 50-km-
wide zone (Berberian and Berberian, 1981). The arc is assumed to have resulted due
to the collision of the Arabian and Central Iranian continental plate margins. The
apparent thickness of the continental crust along this zone is 45-50 km. This increase
in thickness is presumably due to magmatic activity and northeastward thrust faulting
(Alavi, 1994). Seismic data suggest the possibility of the existence of a segment of the
oceanic crust beneath the southeastern part of the Urumiyeh-Dokhtar Magmatic Arc
(Kadinsky-Cade and Barazangi, 1982).
6
1.1.2.3. The Central Iranian Zone
The Central Iranian zone occurs between the Alborz and Kopeh Dagh ranges in
the north and the Zagros and Makran ranges in the west to south and east of Iran. The
Central Iranian crust has been a decoupled part of Africa before becoming part of
Eurasia after the opening of the Neotethys in Triassic times. This microplate, which
formed in pre-Paleozoic times, shows no indication of any Variscan orogeny
(Delaloye et al., 1981). It is fragmented by crustal faults (the Great Kavir, Nain–Baft,
and Harirud faults) into several blocks. The blocks are partly bounded by the Upper
Cretaceous-to-Lower Eocene ophiolite and ophiolitic mélange (Takin 1972). From
east to west, three major crustal blocks can be distinguished (Fig. 2): the Lut Block,
the Tabas Block, and the Yazd Block (Berberian, et al., 1981). The Tabas and Yazd
blocks are separated by a 600-km-long and relatively narrow belt (the Kashmar-
Kerman tectonic zone). The Lut block, which is composed of Paleozoic to Mesozoic
rocks has different lithology from those of Central Iran (Crawford, 1972; Stöcklin,
1974) and may therefore be related to the microplate of Afghanistan-Pamir
(Krumsiek, 1976; Gealey, 1977). It probably collided with Central Iran (Eurasia)
during the Paleogene. The northern part of the Lut Block is covered by Neogene
andesitic, dacitic, and rhyolitic lava flows. According to Alavi (1991a), Central Iran
(Fig. 2) comprises the Sabzevar block and the Tabriz Qom belt (Fig. 3). The Sabzevar
block is characterized by the presence of ophiolitic mélange, which tapers out toward
the east and continues to the transitional Binalud zone. This block is limited by the
Doruneh and Nain Faults from south and east, respectively. Its northern boundary is
not always clear, but in most cases is followed by the Mayamey- Attari fault.
1.1.2.4. The Alborz Zone
The Alborz Mountains, forming a gently sinuous east–west range in the north of
Iran is bounded to the north and northeast by the Paleotethys collision suture zone
(Arabia–Eurasia collision from the Cimmerian orogeny), which is overprinted by the
later Alpine structures. The strong positive gravity anomaly in the southern Caspian
Sea area testifies perhaps to its being a remnant of Paleotethys. The suture zone is
characterized by the presence of metamorphosed Cimmerian ophiolites and deep-sea
sediments. It is affected by Pre-Jurassic structural deformations, which produced
7
intense multiple fold and thrust faults (Alavi, 1979, 1991b). According to Stampfli et
al. (1991), the Alborz zone separated from Gondwanaland in the Ordovician–Silurian
and collided with Eurasia in the Late Triassic (Sengör, et al., 1988). Metamorphic
relics of Palaeotethyan collision are preserved in discontinuous outcrops along the
northern margin, such as the Gorgan Schists in the northeast.
Fig.3. Index map showing the main tectonic elements of Iran (after Alavi, 1991a). Abbreviations: AB =
Alborz belt, AMA = Alborz magmatic assemblage, EIB = East Iranian block, EIMA = East Iran
magmatic assemblage, KD = Kopeh Dagh, LU = Lut block, MAP = Makran accretionary prism, PBB
= Posht Badam block, SB = Sabzevar block, TB = Tabas block, TQB = Tabriz Qom belt, UDMA =
Urumiyeh Dokhtar magmatic assemblage, YB = Yazd block, ZO = Zagros orogen.
1.1.3. The northern unit
The northern unit includes remnants of the Paleotethys and marginal band of the
Variscan realm of Central Asia, mainly overlapped by the Alpine realm. This unit is
sharply separated from the central unit by the north Iran suture. It is identified by
continental crust, including fragments of more or less cratonized former (Paleozoic)
8
oceanic crust. It was deformed and largely consolidated during the early Cimmerian
and late Alpine movements (Stöcklin, 1977). The northern unit includes mainly (1)
the South Caspian depression, (2) the northern part of the Iranian suture zone, (3) the
Kopeh Dagh range, (4) the Paropamisus and (5) the Western Hindu Kush ranges.
Here, only the Kopeh Dagh range is discussed.
The Kopeh Dagh range in northeast Iran is a fold zone marginal to the Turan. It is
formed by Variscan metamorphosed basement with platform sediments in the
southwest. Similar to that in Zagros, this belt is composed of about 10 km of
Mesozoic and Tertiary sediments (mostly carbonates) and was folded during Plio–
Pleistocene.
1.2. Distribution of Basement Rocks in Iran
Consolidation of the Iranian basement units is assumed to occur during
―Baikalian‖ or Pan–African orogeny with post–Pan–African magmatism (e.g.,
Berberian and King, 1981). The basement rocks include both metamorphosed and
unmetamorphosed sedimentary (slightly metamorphosed) and igneous (mostly
granitic) rocks. Metamorphic PT conditions of these rocks range from very-low-grade
to amphibolite (locally granulite) facies.
1.2.1. The Southern Unit
Basement rocks are not exposed on the surface in the southern unit. Only
numerous ―exotic‖ blocks of igneous and metamorphic rocks are present within the
salt diapirs. Low-grade metamorphosed Upper Cretaceous flysch-like sediments with
ophiolitic mélange occur in the Ratuk complex in the eastern Iranian Range (Fig. 4.
A, Geological Survey of Iran, 1990a,b), The mélange with talc–schist, chlorite–schist,
mica–schist with greenschist matrix contain lenses of eclogites, blueschists, and
metacherts (Fotoohi Rad 2005). Considering Tirrul et al., (1983), metamorphism of
these rocks occurred before the Maastrichtian.
1.2.2. TheCentral Units (Cimmerian Block)
Metamorphism and age relations of basement rocks in the Central unit have not
been properly studied, and only a little data are available based on stratigraphical
9
sequences. In addition to metamorphic rocks and age relations of their sedimentary
protoliths, we describe here magmatic rocks occurring in the basement units.
Fig.4. Distribution of metamorphic rocks in southern unit (A), central unit (B-L) and north unit.
Abbreviations: Ag = Aghdarband, AJ =Anarak-Jandagh , Ar = Ardakan, As =Abass Abad, At =
Attari Fault, Az = Azna, Bg = Bafgh, Bi = Biabanak, Bs =Bandar Abass , Bt =Baft, Dd = Dorud, Do
=Doruneh Fault, Ds = Deh Salm, Fa = Fariman, Es = Esphandagheh, Gd =Gonabad, Gol =
Golpaygan, Gr = Gorgan, Hd = Hamadan, Hj = Haji Abad , Kh = Khoy, Kr =Kerman, Lj = Lahijan,
Md = Mashhad, Mh =Masuleh, Mk = Maku, Mo =Moalleman, Mu = Muteh, My =Mayamey Fault, Na
= Nakhlak, Ni = Nain, Nz = Neyriz, Pst = Poshteh Badam, Sa = Saghand, Sdj = Sanandaj, Shz =
Shiraz, Sj = Sirjan, Sq = Saqqez, Sz = Sabzevar, Ta = Tabas, Td = Torud, Tk = Takab, Ur =Urumiyeh,
Yz = Yazd, ZJ = Zanjan.
1.2.2.1. The Sanandaj Sirjan Metamorphic Zone
Metamorphic rocks are exposed in three regions (for simplification marked as B,
C, and D in Fig. 4) along the Sanandaj Sirjan zone.
10
Region B
In the Maku area (Fig. 4. B), slightly metamorphosed Ordovician shales and
sandstones with volcanic rocks are present (Berberian1976, Berberian and Hamdi,
1977). Numerous granitoids, exposed within lower greenschist facies metamorphic
rocks west of the Saqqez region, were previously mapped as Precambrian (e.g.,
Eftekhar-Nezhad, 1973). However, Omrani and Khabbaznia (2003) assumed a
Cretaceous or even younger age for these rocks. U-Pb data from zircon yielded an age
of 551 ± 25 Ma for the Sheikh Chupan granodiorite and 544 ± 19 Ma for the
Bubaktan foliated granite (Hassanzadeh, et al., 2008).
In addition to greenschist-amphibolite facies rocks, the Khoy area (Fig. 4. B)
contains poly-metamorphic ophiolite (lherzolitic, harzburgite and ultramafic meta-
cumulate) with blueschist (Khalatbari-Jafari 2003). Medium-pressure metamorphic
rocks correspond to green schists, metavolcanics, micaschists, amphibolites, and
gneisses. K-Ar age data for amphiboles, biotites, and muscovites in high-pressure
metamorphic rocks yielded 197 Ma to 160 Ma, but Middle Juriassic ages of 158.6 ±
1.4 Ma and 154.9 ±1.0 Ma were obtained for using Ar-Ar data for amphibolite facies
rocks (Ghazi et al. 2001). According to Khalatbari-Jafari (2003), at least four
metamorphic episodes can be based on age data distinguished: Lower Jurassic (197–
181 Ma), Middle Jurassic (160–155 Ma), Lower Cretaceous (121.2 ± 6.2 – 102.1± 5.4
Ma) and Upper Cretaceous (81.2 ± 1.2 – 69.4 ± 1.6Ma). U-Pb data of zircon from the
Moghanlou granitic orthogneiss (between Zanjan and Takab, Fig. 4. B) yielded ages
of 548 ± 27 Ma and 568 ± 44 Ma (Hassanzadeh, et al., 2008).
The very-low-grade sedimentary rocks (shale and siltstone) of the Soltanieh
Mountains in the Zanjan area (Fig. 4. B) are intruded by granite (Alavi-Naini, 1994;
Stöcklin et al., 1965; Stöcklin and Eftekhar-Nezhad, 1969; Babakhani and Sadeghi,
2005). The whole-rock Rb-Sr age of 645 Ma, (Crawford, 1977) and U-Pb zircon ages
of 544 ± 29 Ma and 599 ± 42 Ma were obtained for this granite (Hassanzadeh, et al.,
2008). The granite is foliated, and Rb-Sr data from biotite indicate a Lower- to
Middle- Jurassic age of 175 ± 5 Ma for its metamorphism (McDougall and Harrison,
1999). Younger ages of 53.4 ± 0.4 Ma and 2.8 ± 0.1 Ma were obtained using U-Pb
zircon age for equi-granular biotite granite and leucogranite, respectively
(Hassanzadeh, et al., 2008).
11
Region C
In the central part of the Sanandaj Sirjan zone in the Hamadan area (Fig. 4. C),
metamorphic rocks are derived both from sedimentary and magmatic rocks (the
Alvand granitoid complex). The metasedimentary rocks range from very-low-grade
slate through phyllite, garnet-staurolite schists to andalusite/sillimanite gneisses.
Similar to other units protoliths of metamorphic rocks were assumed to be
Precambrian in age (Furon, 1941), but based on the new geological map (1:100,000
geological map of Hamadan), they are assigned to Midle-Jurassic. K-Ar data from
amphiboles, muscovites, and biotites in these rocks gave ages between 114 Ma to 60
Ma, and granite emplacement occurred during the Late Cretaceous (ca. 82 Ma,
Baharifar et al., 2004).
The area north of Dorud and Azna (Fig. 4. C) contains metamorphosed Late
Paleozoic to Mesozoic succession dominated by metacarbonate, schist, and
amphibolite. They contain andesitic lavas and pyroclastic rocks that are conformably
overlain by Aptian-Albian limestone.
Metamorphic rocks in the Muteh area (Golpaygan, Fig. 4. C) are represented by
gneiss, marble, amphibolite, mica schist, phyllite, and quartzite. The supposed
Precambrian age of this basement unit (Thiele, 1966; Thiele et al., 1968) was doubted
by Rachidnejad-Omran et al. (2002), who assumed a Paleozoic age for volcano-
sedimentary protoliths and a Mesozoic age for their metamorphism. The Muteh
leucogranite occurring in this basement unit yielded an age of 578 ± 22 Ma and 596 ±
24 Ma using the U-Pb method on zircon. Younger K-Ar ages on biotite (128.3 ± 4.7,
68.9 ± 1.0, and 67.5 ± 1.0 Ma) and whole rock (124.4 ± 4.4 Ma) rocks are attributed
to metamorphic overprint in granite (Rachidnejad-Omran et al., 2002). Moritz et al.,
(2006) obtained two Ar-Ar plateau ages of 109.65 ± 2.04 Ma and 86.55± 1.00 Ma for
these rocks.
A large dome of high-grade metamorphic rocks with biotite granite orthogneisses is
exposed in the Haji Qara mountain (Varzaneh near Muteh, Golpaygan area). U-Pb
zircon data yielded an age of 588 ± 23 Ma for granite formation (Hassanzadeh, et al.,
2008). The Middle Jurassic age (174.5 ± 2.9 Ma) of metamorphism of these rocks is
confirmed by K-Ar data on amphibole (Rachidnejad-Omran et al., 2002). A younger
Ar-Ar age of Eocene times is interpreted as the cooling age of this unit (Rachidnejad-
Omran et al., 2002; Moritz et al., 2006).
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Region D
The Tutak gneiss dome (300 km northeast of Shiraz, Fig. 4. D), in the
southeastern part of the Sanandaj Sirjan zone, consists of metagranite and marble
from Kuh-e-Sefid (Alizadeh, 2008) that come from a Devonian-Carboniferous
sequence (Hushmandzadeh et al., 1990). Ar-Ar ages of 179.5±1.7Ma and 76.8±0.2
Ma (Sarkarinejad et al., 2009) are attributed to deformation and metamorphism of
these rocks, respectively.
The Quri-Kor-e-Sefid (Neyriz area, Fig. 4. D) is comprised of low- to high-grade
metamorphic rocks that are thrust over the Neyriz ophiolites (Ricou, 1974). These
metamorphic rocks consist of orthogneiss with some amphibolite lenses in the base
with 520 Ma age (based on U-Pb zircon). In addition to orthogneisses and
amphibolites, migmatites, minor micaschist, marble, metamorphic mafic, and
ultramafic rocks are also present. Based on coral and Palynomorphs, Devonian to
Early Carboniferous ages are assumed for protoliths of meta-sedimentary rocks
(Roshan Ravan et al., 1995; Eshraghi et al., 1999). K-Ar age on amphiboles and micas
are clustered in four groups between 300 and 60 Ma (Sheikholeslami et al., 2003).
These low- to high-grade metamorphic rocks are unconformably overlain by Upper
Triassic rocks (Sheikholeslami et al., 2008).
The Seh-Ghalatoun HP-LT metamorphic with ophiolite mélange is exposed near
the village of Deh Chah in the eastern part of the Neyriz (Fig. 4. D). The predominant
rocks in the area are amphibolite, garnet amphibolite, quartzo feldspathic gneiss,
kyanite gniess, biotite gneiss, muscovite-garnet schist and phyllite with numerous
intercalations of eclogite facies rocks. A Lower Jurassic age of 187 ± 2.6 Ma (based
on zircon SHRIMP U-Pb) and 180 ± 21 Ma (based on monazite CHIME U-Th-total
Pb) is assumed for metamorphism of the Chah-Sabz and Seghalaton rocks (Fazlnia et
al., 2007).
Blueschist facies rocks are 12 km west of Neyriz. The blueschist facies consists of
marble, amphibolite, serpentinized harzburgite and gabbros. K-Ar data on amphibole
yielded Cretaceous age 89.09 ± 1.34, 90.18 ± 0.88 Ma and 91.23 ± 0.89 Ma
(Sarkarinejad et al., 2009) for the blueschist facies metamorphism. A relatively older
age of 119.95 ± 0.88 Ma and 112.58 ± 0.66 Ma for these rocks was obtained using the
Ar-Ar method on biotite. These Late Aptian ages are related to early thrusting and
peak high-pressure metamorphism of the Neyriz ophiolite. Biotite and hornblende
13
from biotite schist and garnet amphibolite yielded ~ 87 ± ±
(Hynes and Reynolds, 1980).
The Sikhoran complex (Fig. 4. D) in the Esphandagheh area is bounded, by a
blueschist metamorphic belt that gave an age of 80.7 Ma (K-Ar on phengite; Ghasemi
et al, 2002). In the northern part of this complex, various metamorphic rocks of
Paleozoic age, including gneisses, amphibolites, marbles and skarn are present. The
Sikhoran complex (mafic-ultramafic) is tectonically overlain, by amphibolites, marbles,
micaschists and green schists of Devonian age (Sabzehei 1974). In the neighbor
region of Baft, black schists and cherts, belonging to the same series, contain remains
of Achritarchs and Chitinozoaires of Lower Paleozoic age (Cambrian to Ordovician,
Sabzehei, 1977). According to Ghasemi et al, (2002) the K–Ar ages of the
amphibolites from the south of the Sikhoran complex, are 164 to 161 Ma (amphibole).
The gneisses at the southwestern margin of this complex yield 301± 7 Ma (biotite),
and the anatectic granite linked to these gneisses give an age of 330 ±8 Ma
(Muscovite). Agard et al, (2006) reported Ar-Ar ages from phengite for three areas:
they are in the range of 82.38 ± 1-102.26 ± 4 and 89.23 ± 1-109.27 ± 3 Ma for Ashin,
95.81 ± 6-126.51 ± 2 Ma for Seghina, and 87.48 ± 1-102.66 ± 2 and 116.57 ± 9-
118.09 ± 1 Ma for samples from Sheikh Ali.
1.2.2.2. The Central Iranian Zone
Metamorphic rocks exposed in Central Iran occur in four regions (E, F, G and H,
Fig. 4).
Region E
The Deh Salm Metamorphic Complex is divided two major lithostratigraphic
groups. The lower group consists of an alternation of marble, schists, and
amphibolites. The upper group consists of phyllites, mica schists and schists. The
whole metamorphic complex is considered to be Upper Triassic- Jurassic and together
with some older rocks they are metamorphosed during the Post- Jurassic (or at least
Post-Middle Jurassic) (Stöcklin et al., 1972).
14
Region F
Metamorphic rocks from the Saghand area are assumed to represent the oldest
lithostratigraphic formations (Nadimi, 2005; Nadimi, 2007). From bottom to top, they
include the Chapedony, Poshte Badam, Boneh Shurow, and Tashk formations (Fig.
5).
The Chapedony Formation (Fig. 4. E) is assumed to represent a Paleoproterozoic
complex (Hushmandzadeh, 1969; Haghipour and Pelissier, 1977). This age for the
source rocks is assumed also by U-P data from detrital zircon of 2140.9 Ma
(Ramezani and Tucker, 2003) and by Rb/Sr whole rock ages of 2382 Ma (Haghipour,
1974). Metamorphic rocks of this formation are schist, amphibolite, marble, quartzite,
gneiss, ribbon gneiss and migmatite (Haghipour, 1974). Their PT conditions range
from greenschist, through amphibolite, to granulite facies. An Eocene age of 48.2 Ma
has been obtained for metamorphic zircon in metagranite (Ramezani and Tucker,
2003).
Fig.5. Generalized stratigraphic section of the basement complexes in the Saghand region (after
Nadimi 2007).
Metamorphic rocks of the Poshte Badam Formation (Fig. 4. F) involve greenstones,
schists, meta-greywacke, marble, gneisses, amphibolites, pyroxenites, serpentinite,
meta-basalt and meta-conglomerate (Haghipour and Pelissier, 1977). The Boneh
Shurow Formation (Fig. 4. E) is a widely exposed metamorphic unit that forms ridge-
15
shaped outcrops directly next to the Poshte Badam fault (Haghipour and Pelissier,
1977; Ramezani and Tucker, 2003). It exhibits a distinct metamorphic layering
composed of an alternation of pink quartzofeldspathic gneisses, greenish-gray mica
schists, and dark-colored amphibolites. The crystallization age of the granitic
protolith(s) of the Boneh-Shurow gneiss (U-Pb zircon) is in the range from 617 Ma to
602 Ma (Ramezani and Tucker, 2003) and an age of 547.6 ± 2.5 Ma is assumed to
date the lower-amphibolite facies metamorphism.
The Tashk Formation (Fig. 4. F) consists of dark greenish-gray greywackes,
schist, quartzitic schist, quartzite, marble, amphibolite, gneiss, slaty shale, sandstone
locally interbedded with arkosic arenites, argillites, and tuffaceous deposits that occur
sub-parallel to the Boneh–Shurow Formation (Haghipour and Pelissier, 1977). U-Pb
zircon ages from the Tashk Formation range between 627 Ma and 533 Ma (Ramezani
and Tucker, 2003).
According to Samani (1994), the Chapedony, Boneh Shurow and Tashk (Natak)
Formations are of Proterozoic age 874-750 Ma, and they underwent a Mesozoic (180-
220 Ma) dynamothermal and a Cenozoic (52 Ma) hydrothermal metamorphism. An Rb-
Sr model age of 540 Ma was obtained for the Bornavard granite (Fig3. F) (Crawford
1977).
Region G
The Anarak Metamorphic Complex, south of Nakhlak (Fig. 4. G), is subdivided
into several subunits (Sharkovski et al. 1984). It consists of phyllites, mica schists,
greenschists, marble, calcareous schists, and blueschists (Sharkovski et al. 1984). A
few remnants of archaeocyathids in marble (Geological Survey of Iran, 1984a) give
an Early to Middle-Cambrian age, whereas crinoids and gastropods dubitatively
suggest a younger age (Sharkovski et al. 1984). Reyre & Mohafez (1970) obtained a
203 ± 13 Ma Rb–Sr mineral age and a Precambrian (845 Ma) whole rock Rb–Sr age
for protoliths of the rocks. Variscan ages of 315 ± 5 Ma (Rb-Sr) and 333–320 Ma (Ar-
Ar) for metamorphism of the Anarak Metamorphic Complex were obtained by
Crawford (1977) and (Bagheri & Stampfli 2008), respectively.
The Doshakh and the Bayazeh units were previously interpreted as a greenschists-
facies Precambrian or Lower Paleozoic metamorphic basement of the Yazd block
(Sharkovski et al., 1984). Middle Permian–Middle Triassic age for some of these
rocks was obtained based on conodonts (Bagheri and Stampfli, 2008), but Sharkovski
16
et al. (1984) reported a wider range of K-Ar ages (400–180 Ma from metamorphic
rocks in this area. The younger ages are assumed to be related to later retrograde
events (Romanko and Morozov, 1983; Almasian, 1997). K-Ar ages of 270–183 Ma
were obtained by Sharkovski et al. (1984). Two set of Ar-Ar data were obtained by
Bagheri et al. (2007), and most Ar-Ar data from muscovite yielded 333–318 Ma
cooling ages that are related to the first metamorphic event. Two samples of
muscovite and hornblende data gave Middle Jurassic age of 163.86 ± 1.76 Ma and
156.56 ± 33.15 Ma, respectively, for metamorphism of the basement rocks (Bagheri et
al., 2007).
Region H
This region is a part of Central Iranian Block after Stöcklin (1974) and Berberian
and King (1981), and the Sabzevar Block according to Alavi (1991a, Fig. 3). Similar
to other high-grade metamorphic rocks in Central Iran, the rocks exposed in this block
were mapped as a Precambrian basement (geological map, 1: 250,000 scale, the
Geological Survey of Iran). The metamorphic complex in the Khar Turan area (Fig. 4.
H) consists of amphibolite-grade gneisses and schists (Hassanzadeh, et al 2008).
Neoproterozoic ages (554 ± 40, 534 ± 31, 522 ± 23 Ma and 551 ± 20 Ma) were
obtained for granite and protoliths of the orthogneiss, using U-Pb dating on zircon
(Hassanzadeh, et al 2008). The orthogneisses of Band-e Hezar Chāh (Fig. 4. H)
yielded relatively older ages of 581 ± 21 Ma, and 601 ± 22 Ma (Hassanzadeh, et al.,
2008). A sample of biotite-rich granite gneiss in Torud area yielded 566 ± 31 Ma
(Hassanzadeh, et al 2008).
Paleozoic sedimentary and metasedimentary rocks are exposed in the Torud area
(Hushmandzadeh et al., 1978). Based the geological map in the Kalateh-Reshm, the
polymetamorphic and polydeformed rocks with some mafic and ultramafic rocks near
Kuh Siah Poshteh are assumed to be Paleozoic in age (Jafariyan, 2000). Some small
remnants of ophiolitic rocks are exposed near the Khondar anticline in the Meyamey
(Amini Chehragh, 1999), but they are interpreted as Pre-Cretaceous (?) in age. Pre-
Jurassic (Khalatbari Jafari et al., 2008) and Post-Jurassic to Pre-Cretaceous (Navai,
1987) metamorphic rocks are exposed in the Abass Abad and Biarjmand areas.
The Shotur Kuh metamorphic complex was assumed as Precambrian metamorphic
basement (Hushmandzadeh et al 1978). Geological mapping in the Shotur Kuh area
(Rahmati Ilkhchi et al., 2002) as well new petrological and geochrnologicala data
17
show that the orthogneisses are derived from Neoproterzoic granites that were
metamorphosed during the Middle Jurassic.
The western belt of Fariman is a continuation of the Sabzevar ophiolitic belt that
runs parallel to the Mayamey fault and separates central Iran from the Alborz-Binalud
structural zone (Khalatbari Jafari et al., 2009). The Cretaceous Fariman belt
discontinuously runs parallel with the Doruneh fault and joins to the Nain ophiolite.
1.2.2.3. The Alborz Zone
Metamorphic rocks are exposed in three regions (I, J and K) along the southern
border of the Caspian Sea (Fig. 4) and one in the Bilalud area (L).
Region I
The Shanderman Complex (Fig. 4. I) is formed mainly by metabasites and mica
schists, with minor calcschists, quartzites, phyllites and patches of serpentinized
peridotite (Davies et al., 1972; Clark et al., 1975). The whole-rock Rb-Sr data from
phyllite yielded a Middle–Late Devonian age of 382 ± 48 and 375 ± 12 Ma
(Crawford, 1977). However, a Neoproterozoic age of 551 ± 9 Ma was obtained for the
Lahijan biotite granite using the U-Pb data on zircon (Hassanzadeh, et al., 2008; Lam,
2002). An Ar-Ar age of 315 ± 9 Ma from paragonitic white micas in phyllite suggests
a Late Carboniferous metamorphism of these rocks (Zanchetta et al., 2009). Lam
(2002) obtained, using Th/He data on zircon, three ages of 133 ± 7.98 Ma, 152 ± 9.12
Ma and 162 ± 9.72 Ma that are interpreted to represent Middle Jurassic to Early
Cretaceous cooling of these rocks.
Region J
The Kahar Formation (Fig. 4. J) is assumed to be Neoproterozoic in age (e.g.,
Gansser and Huber, 1962; Stocklin et al., 1969; Stocklin, 1974, 1977; Jenny, 1977).
However, Upper Riphean to Middle-Upper Vendian or Paleozoic age for formation
was assumed based on fossils (Sabouri, 2003., Golshani, 1990).
Region K
The Gorgan metamorphic rocks (Fig. 4. K) involve mainly phyllites, sericite-
chlorite-schists, and quartzite. They show low-grade metamorphic conditions with
18
prehnite-pumpellyite and have been considered Precambrian in age (Afshar-harb,
1979; Delaloye, et al., 1981; Gansser, 1951; Hubber, 1957; Salehi-Rad, 1979; Stahl,
1911; Stampfli, 1978; Stöcklin, 1971; Tietze, 1877). A Late Ordovician (Caradoc-
Ashgill) age for these rocks is based on fossils (Ghavidel Syooki 2007).
Region L
Metamorphosed Late Paleozoic-Devonian rocks are exposed in the Binalud area
(Fig. 4. K, Majidi, 1978). According to Davoudzadeh et al. (1975), metamorphism of
the Rhaetic-Liassic sediments of the Binalud area occurred during Middle Jurassic
(around 176 Ma). Four main deformation stages are determined in this area.
Deformation D1 and D2 correspond to progressive stage of metamorphism that
occurred in greenschist to amphibolite facies conditions. D3 deformation is related to
retrogression, and D4 occurred in relatively brittle condition and relates to younger
metamorphic process (Koohpeyma, 2007).
1.2.3. The Northern Unit
Metamorphosed Late Paleozoic-Devonian rocks are exposed at Aghdarband in the
Kopeh Dagh (Majidi, 1978).
1.2.4. Summary of Geochronological Data of Metamorphism
in Basement Rocks in Iran
Consolidation of the Iranian basement units by metamorphism with partial
granitization and partly intense folding occurred during Late Precambrian. This
process continued during several major orogenic phases that were accompanied by
syntectonic metamorphism and magmatic events (Aghanabati, 2004). The most
important orogenic events were during the Paleozoic, Middle Triassic, Middle
Jurassic, Late Jurassic and late Cretaceous periods (Fig. 6). Some authors (e.g.,
Hushmandzadeh el al., 1973) assume only epeirogenic movements during Paleozoic
time. However, available geologic and age data (Fig. 7, Table. 1) suggest the
occurrence of Variscan (300 -333Ma) or/and Caledonian (375-400Ma) deformation
with questionable regional metamorphism, although several orogenic events
accompanied with metamorphism have now been established during the Mesozoic.
19
The earliest Alpine events were apparently vigorous in some parts of Iran, and they
were accompanied by metamorphism (Hushmandzadeh et al., 1973).
Fig.6. Classifications
of orogeny and
epeirogeny activity,
which affected the
geology of Iran in
different times (after
Aghanabati, 2004)
20
The Cimmerian events occurred between 200 Ma and ca. 65Ma and correspond
well to the closure of the Neo-Tethyan basin. The Early Cimmerian event occurred
during the Middle Triassic; causing the paleogeographical changes, accompanied by
folding, magmatic intrusions, and metamorphism in the Paleozoic platform cover and
in Early Mesozoic sediments (Hushmandzadeh, el al. 1973). The Late Cimmerian
orogeny occurred at the Jurassic-Cretaceous boundary, and it was previously thought
that this orogeny was accompanied by folding, paraconformity, magma emplacement,
and metamorphism, but it is now believed that all of these orogenic phenomena must
be associated with the Mid-Cimmerian (Bathonian/intra Bajocian, Aghanabati, 1992).
Evidence for a Mid-Cimmerian movement (~154.9-176 Ma) in many parts of Iran
includes tectonic uplift accompanied by denudational interval, paraconformity,
igneous activates (Shirkuh granite, Kolaghazi granite, Shahkuh granite and
Charfarsakh granite, Aghanabati, 1992), and metamorphism. Successive Early
Tertiary movements accompanied a weak regional metamorphism in this period. The
latest orogenic episode with metamorphic overprint occurred between the Eocene and
Oligocene time.
Geochronological data summarized in Table 1 and Fig. 7 show two maxima with a
high number of Cretaceous (60-130 Ma) and fewer frequent Upper- to Middle-
Jurassic (155-165 Ma) ages of metamorphism for the Sanandaj Sirjan zone, In
contrast, the Central Iran basement suffered mainly by Jurassic metamorphism. There
are only a few Jurassic-to-lower Cretaceous metamorphic ages in Albroz. The
concentration of Jurassic ages in Central Iran and partly in Sanandaj Sirjan and
Albroz suggests that this metamorphism was significant for the entire regions.
Generally, the data show youngering of metamorphic recrystallization and mineral
formation age from north to the south.
21
Fig.7. Histogram of available radiometric ages of metamorphic minerals from Alborz, Central Iran and Sanandaj Sirjan. The age data and location of the samples
are in Figure 4, and Table 1.
22
Table 1. Summary of Available Geochronological Data from Metamorphic Basement Rocks in Central Iran, Including the Sanadaj Sirjan zone, Albroz and Shotur Kuh
----------------------------------------------------------------------------------------------------------------------------------------------------
Subunit rock Protolith age Age of metamorphism
Fossil/U-Pb U-Pb K-Ar Ar-Ar reference
Sanandaj Sirjan zone
Region B granitoids 551 ± 25 Ma (17)
granitoids 544 ± 19
Micaschist, Amphibolite
Gneiss, meta-ophiolite
197 Ma (20)
160 Ma
158.6± 1.4 Ma (14)
154.9 ±1.0
197Ma (20)
181 Ma (20)
160Ma (20)
155 Ma (20)
121.2 ± 6.2 Ma (20)
102.1± 5.4 Ma (20)
81.2 ± 1.2 Ma (20)
69.4 ± 1.6Ma (20)
granitic orthogneiss 548 ± 27 Ma (17)
568 ± 44 Ma (17)
granite 645 Ma* (6)
granite 544 ± 29 Ma (17)
599 ± 42 Ma (17)
foliated granite 175 ± 5 Ma* (23)
biotite granite 53.4 ± 0.4 Ma (17)
23
leucogranite 2.8 ± 0.1 (17)
Region C (17)
metasedimentary Precambrian 114 Ma (5)
L-M Jurassic 60 Ma (5)
82 Ma (5)
Gneiss,marble,Amphibolite Precambrian (37)
Paleozoic 128.3 ± 4.7 Ma (25)
leucogranite 578 ± 22 Ma 68.9 ± 1.0 Ma 109.65 ± 2.04 Ma (17), (25), (24)
596 ± 24 Ma 124.4 ± 4.4 Ma* 67.5 ± 1.0 Ma) 86.55± 1.00 Ma (17), (25), (25),
(24)
granite 588 ± 23 Ma 174.5 ± 2.9 Ma (17), (25)
Region D (25)
metagranite, marble Devonian-
Carboniferous
179.5±1.7Ma (19), (33)
76.8±0.2Ma (33)
orthogneiss amphibolite 520 Ma (35)
meta-sedimentary Devonian to early
Carboniferous
300 Ma (29), (8), (35)
60 Ma (35)
HP-LT metamorphic rock 187±2.6 Ma 89 (9), (33)
180±21 Ma (9), (33)
(33), (18)
(18)
marbles, micaschists Cambrian to
Ordovician
164 to 161 Ma (31), (13)
gneisses 301± 7 Ma (13)
anatectic granite 330 ±8 Ma (13)
HP-LT metamorphic rock 82.38±1 Ma (2)
102.26±4 Ma (2)
24
89.23±1 Ma (2)
109.27±3 Ma (2)
95.81±6 Ma (2)
126.51±2 Ma (2)
87.48±1 Ma (2)
102.66±2 Ma (2)
116.57±9 Ma (2)
118.09±1 Ma (2)
Central Iran
Region E
meta-sedimentary Upper Triassic-
Jurassic
(36)
Region F
schist,amphibolite, marble 2382 Ma*
2140.9 Ma
48.2 Ma (16), (27)
granite 617 Ma 547.6 ± 2.5 Ma (27)
602 Ma (27)
schist, quartzite, marble,
amphibolite, gneiss
627 Ma (27)
533 Ma (27)
874 Ma 220 Ma (32)
750 Ma 180Ma (32)
52 Ma (32)
Granite 540 Ma* (6)
Region G
phyllites, mica schists,
greenschists, blueschists
Early–? Middle
Cambrian age
845 Ma* 203 ± 13 Ma* 333–320 Ma (28), (28), (4)
315 ± 5 Ma* (6)
meta-sedimentary Middle Permian– 333–318 Ma (6)
25
Middle Triassic
400 Ma. (34)
180 Ma. 163.86±1.76 Ma (34), (3)
156.56 ± 33 Ma (3)
270 Ma (34)
183 Ma (34)
Region H gneisses and schists 554 ± 40 Ma (17)
534 ± 31 Ma (17)
522 ± 23 Ma (17)
551 ± 20 Ma (17)
581 ± 21 Ma (17)
601 ± 22 Ma (17)
granite gneiss 566 ± 31 Ma (17)
Alborz
Region I
metabasites , micaschists,
peridotite
375±12 Ma* (6)
382±48 Ma* 315+9 Ma (6), (38)
granite 551 ± 9 Ma 133 + 7.98 Ma (17), (21)
152 + 9.12 Ma (21)
162 +9.72 Ma (21)
Region J phyllites, schists Neoproterozoic (10)
U- Riphean to M-
U- Vendian
(30)
Paleozoic (15)
Region K
phyllites, sericite-chlorite-
schists
Precambrian (1)
Late Ordovician (12)
26
Region L
meta-sedimentary Late Paleozoic-
Devonian
176 Ma (22), (7)
Shotur-Kuh
orthogneiss 547 ± 7 Ma 141.2 ± 2.2 166 Ma (26)
566 ± 31 Ma 208.7 ± 3.2 (17), (26)
171.8 ± 2.7 (26)
(1) Afshar-harb (1979), (2) Agard et al (2006), (3) Bagheri et al (2007), (4) Bagheri & Stampfli (2008), (5) Baharifar et al (2004), (6) Crawford (1977), (7) Davoudzadeh et
al (1975). (8) Eshraghi et al (1999), (9) Fazlnia et al (2007), (10) Gansser and Huber (1962), (11) Geological Survey of Iran (a,1984), (12) Ghavidel Syooki (2007), (13)
Ghasemi et al (2002), (14) Ghazi et al (2001), (15) Golshani (1990), (16) Haghipour (1974), (17) Hassanzadeh, et al (2008), (18) Hynes and Reynolds (1980), (19)
(Hushmandzadeh et al 1990), (20) Khalatbari-Jafari, et al (2003), (21) Lam (2002), (22) Majidi (1978), (23) McDougall and Harrison (1999), (24) Moritz et al (2006), (25)
Rachidnejad-Omran et al (2002), (26) Rahmati Ilkhchi et al (submitted for publication), (27) Ramezani and Tucker (2003), (28) Reyre & Mohafez (1970), (29) Roshan Ravan
et al (1995), (30) Sabouri (2003), (31) Sabzehei (1977), (32) Samani (1994), (33) Sarkarinejad et al (submitted for publication, 2009), (34) Sharkovski et al (1984), (35)
Sheikholeslami et al (2003), (36) Stöcklin et al (1972), (37) Thiele (1966), (38) Zanchetta et al (2009).
540 Ma* = whole rock (Rb-Sr)
27
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CHAPTER 2
Magmatic and metamorphic evolution of the Shotur Kuh
Metamorphic Complex (Central Iran)
Mahmoud Rahmati-Ilkhchi
1,2, Shah Wali Faryad
1, František V. Holub
1, Jan Košler
3,
and Wolfgang Frank4
1 Institute of Petrology and Structural Geology, Charles University in Prague, Albertov 2, 128 43
Prague, Czech Republic
2 Geological Survey of Iran
3 Department of Earth Science and Centre for Geobiology, University of Bergen, Allegaten 41, N-5007
Bergen, Norway
4 Central European Argon Laboratory, Slovak Academy of Sciences, Valasška, Bratislava
International Journal of Earth Sciences (accepted)
Abstract
Metamorphic basement rocks, that are exposed beneath the very-low-grade to
unmetamorphosed Upper Jurassic-Eocene formations north of the Torud fault zone
within the Great Kavir Block, were investigated in order to elucidate origin of their
protoliths and the pressure and temperature conditions of metamorphism. The
basement, previously assumed as a pre-Cambrian metamorphic complex, is mostly
formed by amphibolite facies orthogneisses (tonalite, granodiorite, and granite) with
amphibolites and small amounts of metasediments-micaschists. Major- and trace-
element geochemistry in combination with U-Pb age dating of zircon showed that the
protoliths formed during Late Neoproterozoic continental arc magmatism that has also
been identified in other tectonic blocks of Central Iran. In addition to quartz,
feldspar(s), micas in orthogneisses, and amphibole + plagioclase in amphibolite, all
rocks may contain garnet that shows prograde zoning. Kyanite was found only in
some Al-rich amphibolite together with gedrite. The PT conditions of the rocks, based
conventional geothermobarometry and the pseudosection method, show a medium-
pressure amphibolite facies metamorphism. Ar-Ar age dating of muscovite reveals
that this metamorphism occurred 166 Ma ago (Middle Jurassic) and related to the
closure of the Neotethyan basin.
--------------------------------------------------------------------------------------------------------
Keywords: Late Neoproterozoic arc magmatism, Jurassic barrovian type
metamorphism, Central Iran
38
2.1. Introduction
The Central Iranian Terrane is a composite of fold-and-thrust belts with basement
blocks covered by thick sedimentary sequences. Comparative stratigraphy and
tectonics, combined with geochemistry of igneous rocks in Central Iran, led to the
identification of several crustal blocks (e.g., Stöcklin, 1968, 1977; Takin, 1972;
Crawford, 1977; Berberian et al., 1981). Ramezani and Tucker (2003) summarized
tectonic and geochronological data from Central Iran and focused on igneous rocks in
the Saghand area that previously had been thought to represent the Precambrian
basement. The igneous/metaigneous rocks occur along a north-south-trending belt, the
Kashmar-Kerman Tectonic Zone (KKTZ), which separates the Yazd block to the west
and the Tabas block to the east in Central Iran (Fig. 1a). Based on U-Pb ages and
geochemical data, Ramezani and Tucker (2003) postulated that these rocks are
remnants of a Late Neoproterozoic arc that developed along the Proto-Tethyan margin
of the Gondwanaland supercontinent. Metamorphism of these rocks that reached
amphibolite facies conditions occurred in Eocene times. However, several basement
rocks exposed beneath the Paleozoic-to-Mesozoic sedimentary sequences west of the
KKTZ are still assumed to be Pre-Cambrian metamorphic basement, even though
detailed information about their composition and metamorphic evolution is lacking.
Bagheri and Stampfli (2008) investigated amphibolite facies metaigneous and
metasedimentary rocks exposed along the boundary between the Yazd and Great
Kavir blocks and postulated a Paleozoic accretionary complex with a 320 Ma age of
greenschist-to-amphibolite facies metamorphism and a Permian-Triassic (280–230
Ma) high-pressure metamorphism. It seems that boundaries between different crustal
blocks were significant in the formation of igneous and metamorphic rocks during the
geological evolution of Central Iran.
In this work, we focus on the Shotur Kuh metamorphic complex in the Great
Kavir Block (GKB) (Fig. 1a). It forms a tectonic window beneath the Mesozoic and
Cenozoic sediments near the Torud fault zone. Similar to other crystalline windows
exposed in Central Iran, it has been assumed to represent a Pre-Cambrian
metamorphic basement. In addition to geochemistry and metamorphic evolution, we
present new U-Pb zircon and Ar/Ar mica ages. The implications of our results for the
metamorphic and tectonic evolution of the GKB along the Torud tectonic zone, as
39
well as relationships to other igneous and metamorphic complexes in Central Iran, are
discussed.
2.2. Geological setting
From east to west, the Central Iranian Terrane consists of four major crustal
domains (Fig. 1a): the Lut Block, the Tabas Block, the Yazd Block and the Great
Kavir Block (see for example, Berberian et al., 1981; Bagheri and Stampfli, 2008).
These blocks are separated by a series of intersecting regional-scale faults. Although
the stratified cover rocks are largely comparable among different blocks, locally
significant facies and/or thickness variations occur across the domain boundaries.
Each block features a particular overall deformation style and pattern of recent
seismicity, distinguishable from those in the adjacent domains (Berberian, 1981). The
Tabas and Yazd blocks are separated by a nearly 600-kilometer-long, arcuate, and
structurally complex belt, the KKTZ, composed of variably deformed and fault-bound
supracrustal rocks.
The Rezveh area with the Shotur Kuh metamorphic complex (SKMC) is a part of
the Great Kavir Block (GKB) and occurs north of the Torud fault zone (Fig. 1b). This
tectonic zone is followed by a series of Eocene volcanic rocks of dacitic and andesitic
composition. The SKMC represents an E-W-trending elliptical tectonic window (c. 20
km long and 11 km wide) that, together with its Pre-middle-Triassic tectonic cover, is
exposed beneath the Jurassic-Miocene sedimentary sequences (Fig. 1c). Most of the
basement rocks have igneous precursors, and only small amounts of micaschists and
phyllites, which represent the lower part of the Pre-middle Triassic (Permian-Lower
Triassic?) sequence with metamorphosed limestone and dolomite are present. These
rocks occur along the southern and northern boundaries of the SKMC. The main rocks
are orthogneisses of tonalitic, granodioritic, and granitic compositions with various
amounts of amphibolites (probably former dykes) that have lengths up to several tens
of meters and widths from centimeters to 20 m. The amount and thickness of the
amphibolite bodies decreases from meta-granodiorite to granite, and they are most
common in the central and western part of the complex. In some cases, amphibolite
and orthogneiss may form parallel stripes and bands that are locally deformed into
isoclinal folds. The metagranite occurs along the northern border and in the central
part of this complex.
40
Fig. 1. Simplified tectonic map of eastern Iran showing constituent crustal blocks (compiled from
Ramezani and Tucker, 2003; Alireza Nadimi, 2006). KKTZ- Kashmar-Kerman Tectonic Zone, DBF-
Dehshir Fault, TFZ. (b) Schematic map of the Torud area with important faults (after Hushmandzadeh
et al., 1978). (c) Simplified geological map of the Rezveh area (Rahmati-Ilkhchi, 2002).
Metapelitic rocks (black shale-phyllite-garnet micaschist) rimming the southern
border of the complex form the lower part of the Pre-middle Triassic (Permian-Lower
Triassic?) formation. They intercalate with calcitic and dolomitic marbles that upward
become a massif carbonatic sequence with few or no pelitic rocks. Marbles and garnet
micaschists are isoclinally folded together with metaigneous rocks and are
tectonically juxtaposed with phyllites, which show a lower degree of metamorphism.
The Pre-middle Triassic pelitic-carbonatic rocks are covered by the Upper Triassic-
41
Lower Jurassic Shemshak formation, which is represented by the intercalation of
psammopelitic rocks with conglomerates. They show metamorphic fabrics similar to
those in the Pre-middle Triassic rocks, but in lower greenschist facies conditions that
are characterized by the presence of chlorite, fine-grained white mica and quartz. The
uppermost (Middle Jurassic) part of the Mesozoic sequence, beneath the Neogene
sediments, is characterized by very-low-grade sandstones, shales and conglomerates,
which contain pebbles of the basement rocks (Rahmati-Ilkhchi, 2002).
2.3. Analytical methods
We studied the rocks using bulk-rock geochemistry, mineral composition, and age
dating of zircon and micas. Powdered whole-rock samples were analyzed in the
Activation Laboratories (Actlabs), Ltd. (Ancaster, Ontario) using the lithium
metaborate/tetraborate fusion technique followed by a combination of (1) inductively
coupled plasma emission spectroscopy (ICP-OES) for major elements plus Sc, Be,
and V; and (2) inductively coupled plasma mass spectrometry (ICP-MS) for all other
trace elements. The analytical results (see Table 1) are illustrated in geochemical
diagrams using the GCDkit software (Janoušek et al., 2006).
Mineral chemical analyses were carried out with a CAMECA SX 50 electron
microprobe at the Institute of Mineralogy, Technische Universität Stuttgart, which is
equipped with four wavelength-dispersive spectrometers. The following synthetic
standards were used: pyrope (Si, Al, Mg), andradite (Ca, Fe), jadeite (Na), spessartine
(Mn), K-silicate glass (K), Ba-silicate glass (Ba), and NaCl (Cl), as well as natural
rutile (Ti) and topaz (F). The operating voltage was 15 kV using beam currents
between 10 and 15 nA. The beam was focused to 1–2 μm diameter, except for micas,
for which an 8–10 μm beam was used. The peak counting times were 20 seconds.
Some compositional profiles of garnet were obtained using the scanning electron
microscope JEOL 6310 at the Institute for Petrology and Structural Geology, Charles
University, Prague. Representative mineral analyses are given in Table 2.
U/Pb dating of zircons was performed by laser ablation inductively coupled
plasma mass spectrometry (LA-ICPMS) using a New Wave UP-213 (213 nm,
Nd:YAG) laser system coupled to a Thermo Finnigan Element 2 ICP-MS instrument
at the Department of Earth Science, University of Bergen. The analytical technique
and data reduction were modified from Košler et al. (2002). Ar-Ar age dating was
42
obtained by measuring the 40
Ar*/39
Ar isotopic ratio using the mass spectrometry VG
5400 at the Central European Ar-Laboratory in Bratislava.
Petrography
The main rock groups of the SKMC are orthogneisses that are derived from rocks
of granitic, granodioritic and tonalitic compositions. The metatonalite are
characterized by the presence of amphibolite bodies, and they are exposed in the
central and eastern parts of the complex. Phyllites to micaschists studied here come
from the southern border of the SKMC.
Orthogneisses
The rocks are medium- to coarse-grained (partly) display augen textures with
various degrees of foliation. At least three varieties of orthogneisses can be
distinguished based on the present mineral assemblages: amphibole + plagioclase +
biotite + quartz ± garnet (metatonalite); plagioclase + biotite + quartz ± garnet; and
plagioclase + biotite + muscovite ± garnet (metagranodiorite to metagranite). Grain
size and augen structure in the orthogneisses increase from tonalites to granites. In
addition to feldspars, quartz and biotite, some tonalitic and granitic varieties may
additionally contain amphibole and muscovite, respectively. Zircon, apatite, rutile,
titanite, allanite, and monazite are accessory phases. Metagranites are strongly foliated
and they are rich in muscovite (up to 10 vol %); together with biotite and thin quartz
bands, they define the foliation (Fig. 2a). Such rocks occur along the northeastern
border and also in the central part of this complex. Garnet is not found often but
occurs in all compositional varieties of the orthogneisses. The relict K-feldspar is
slightly perthitic, but microcline and myrmekite can be also observed. An interesting
feature in metatonalites and metagranodiorites is the presence of dark-brown allanite,
which may form up to 1 mm long crystals oriented parallel to the foliation. It is
usually rimmed by epidote.
The rocks show various degrees of retrogression and recrystallization, which is
characterized by the presence of several mineral phases. Large plagioclase crystals
contain small grains of zoisite-clinozoisite, and also, though rarely, of calcite in the
cores. Both feldspars can be partly replaced by fine-grained white mica. Garnet is
usually cracked, and along cracks and rims it is partially replaced by fine-grained
43
biotite and chlorite. The fine-grained variety of biotite is also present in some
mylonitized orthogneisses. Compared with the coarse–grained, red-brown biotite,
which shows textural equilibrium with garnet, the fine-grained variety has light-brown
and green colours.
Fig. 2. Microphotos of an muscovite-bearing orthogneiss metagranite (a) and micaschist-phyllite (d)
from the Shotur Kuh metamorphic complex. (b) and (c) are backscatter images of gedrite and kyanite-
bearing amphibolite.
Amphibolites
Amphibolites are mostly garnet-free, and the major phases are plagioclase and
amphibole, which define the foliation of the rock. Porphyroblastic garnet (up to 5 mm
in size) and quartz can each be present up to 15 vol %. Accessory phases are zircon,
apatite, titanite, and opaque minerals, secondary minerals involve epidote and chlorite
which replace amphibole. Relict igneous clinopyroxene was observed in one sample
(I-10a) and it forms large crystals (1 to 3 mm in size) that are mostly replaced by
actinolite and epidote. Plagioclase crystals (1–2 mm in size) are partly replaced by
sericite.
44
In one case (sample P182), kyanite and gedrite were found (Figs. 2b and c). This
rock is coarse-grained and well foliated, and consists of calcic amphibole, plagioclase,
gedrite, and accessory rutile, kyanite, biotite, and phengite. Gedrite, which is
characterized by crystals up to 7 mm long that show a weak pleochroism from pale to
yellow-grey, forms intergrowths with calcic amphibole and contains inclusions of
plagioclase. Phengite was found in kyanite, which together with partial replacement of
amphibole by chlorite along the cleavage indicates very weak retrogression.
Micaschists
The fine-grained rocks grading from phyllites to micaschists come from the
southern border of the SKMC. The micaschists are strongly foliated and consists of
quartz, white mica, garnet, biotite, and small amounts of plagioclase. In contrast to
muscovite-bearing orthogneisses they contain a very fine graphitic substance that is
indicative of a sedimentary protolith. Garnet crystals (up to 0.5 mm in size) form
idioblasts, with pressure shadows consisting of quartz and large flakes of biotite and
muscovite (Fig. 2d). The rocks are retrogressed to various degrees, with garnet grains
being replaced by calcite and chlorite, biotite being replaced by chlorite, and
plagioclase containing numerous flakes of fine-grained white mica.
2.5. Geochemistry
In order to characterize the tectono-magmatic evolution of the rocks, 16 samples
(10 orthogneisses and 6 amphibolites) were selected for whole-rock analyses.
Representative data are listed in Table 1. Since we did not recognize any particular
evidence for major-element transport during post-magmatic evolution and regional
metamorphism, we assumed that the present compositional variations of the
orthogneisses and amphibolites may largely reflect the original characteristics of the
igneous rocks. No chemical element, however, may be considered as fully
―immobile‖, and therefore all of the geochemical conclusions should be regarded with
caution.
Orthogneisses
Orthogneisses represent a heterogeneous group of subalkaline rocks with SiO2
ranging from 63.7 to 76.9 wt % and variable amounts of alkalis (Na2O 2.8 to 3.9 wt
%, K2O 0.5 to 4.6 wt %).
45
Table 1 Representative whole-rock chemical analyses of the Shotur Kuh orthogneisses and amphibolites. Major oxides are in weight percent, trace elements
in parts per million.
I-10 P8 P160 P109 P122 P203 P147 P183 P108 P202 P182 I-5 P179 P29 P68 P74
Rock Orthogneiss Amphibolite
SiO2 63.72 64.29 71.74 72.21 73.66 73.72 74.54 75.16 75.91 76.89 46.21 49.58 51.12 51.57 49.91 51.6
TiO2 0.57 0.73 0.12 0.44 0.24 0.38 0.19 0.18 0.12 0.06 0.69 0.72 1.65 2.12 2.26 1.40
Al2O3 13.42 16.30 12.11 13.23 13.36 12.71 13.06 13.02 12.27 12.22 18.36 16.35 15.35 14.91 13.73 17.46
Fe2O3 tot. 10.42 5.88 1.67 3.77 2.57 2.80 2.02 1.84 1.54 0.90 8.88 8.63 11.98 13.25 13.41 13.45
MnO 0.18 0.05 0.01 0.04 0.04 0.03 0.01 0.02 0.04 0.01 0.12 0.12 0.18 0.20 0.21 0.22
MgO 1.74 3.06 0.40 0.81 0.58 0.56 0.39 0.36 0.27 0.13 12.19 8.80 6.95 5.75 5.32 2.49
CaO 5.68 2.22 2.81 3.13 1.63 1.43 0.91 1.56 1.15 0.44 8.65 11.47 9.59 9.29 9.19 8.29
Na2O 3.21 3.88 3.19 3.39 3.36 2.89 3.43 3.48 2.78 3.21 2.80 2.68 2.27 1.63 2.65 2.44
K2O 0.47 2.47 3.35 1.33 3.23 4.61 4.30 3.86 4.08 4.59 0.20 0.53 0.31 0.43 1.69 1.33
P2O5 0.06 0.22 0.59 0.12 0.08 0.11 0.23 0.05 0.05 0.06 0.09 0.07 0.23 0.30 0.34 0.45
LOI 0.30 1.09 2.86 0.89 1.09 0.56 1.20 0.28 0.79 0.68 1.62 0.90 1.20 1.47 1.11 0.61
Total 99.20 100.20 98.86 99.36 99.83 99.81 100.30 99.81 98.98 99.19 99.63 99.85 100.80 100.90 99.82 99.74
Mg 24.9 50.8 32.2 29.9 30.9 28.4 27.7 27.9 25.8 22.3 73.1 66.9 53.5 46.2 44.0 26.8
A/CNK 0.83 1.24 0.87 1.04 1.11 1.03 1.09 1.02 1.11 1.11 0.89 0.63 0.71 0.74 0.60 0.85
MALI -2.0 4.1 3.7 1.6 5.0 6.1 6.8 5.8 5.7 7.4 -5.6 -8.3 -7.0 -7.2 -4.9 -4.5
Rb 5 100 86 49 104 215 107 128 131 267 8 14 6 18 54 50
Sr 287 247 139 240 110 65 82 73 104 26 207 166 142 206 154 283
Ba 130 603 598 792 720 332 706 305 526 77 36 83.2 49 103 126 389
Y 23.1 35.4 33.0 14.2 23.6 59.6 32.8 46.2 21.7 52.0 19.0 17.5 42.3 41.0 46.1 41.3
Zr 609 220 132 215 161 226 127 139 101 64 66 48 127 198 223 209
Nb 11.4 16.4 10.4 7.7 9.3 11.3 6.5 8.8 5.8 6.7 3.0 1.7 7.1 19.5 16.4 14.4
Hf 15.2 5.3 3.7 4.7 4.1 6.4 4.0 4.8 3.1 3.0 1.5 1.5 3.3 4.4 5.9 4.8
Ta 0.6 1.3 0.9 0.4 0.8 1.2 0.9 0.9 1.0 1.8 0.2 0.1 0.4 1.1 1.2 0.8
Th 6.4 12.9 14.8 9.6 14.8 34.9 14.3 17.9 16.1 14.5 0.4 0.7 1.73 2.41 3.4 3.19
La 29.3 30.0 33.6 40.5 30.9 49.1 28.6 22.8 16.2 11.3 4.0 3.6 10.4 17.8 18.2 37.2
Ce 59.2 63.8 68.7 74.5 60.7 105 57.9 49.4 32.8 28.1 10.8 9.6 24.6 40.8 42.7 89.9
Sm 7.00 5.73 5.78 4.07 3.97 8.67 5.54 6.40 2.72 4.23 1.84 2.30 4.43 5.68 7.02 7.75
Eu 1.85 1.19 0.63 1.29 0.61 0.93 0.86 0.44 0.49 0.19 0.80 0.94 1.65 2.00 2.31 2.01
Gd 5.37 5.92 5.79 3.53 3.78 8.14 4.87 6.77 2.89 4.52 2.58 2.73 6.21 7.08 7.28 7.9
Yb 2.39 2.76 2.98 1.13 2.33 5.30 2.95 4.86 2.33 5.66 1.68 1.47 3.59 3.30 4.04 3.46
Atomic ratio mg = 100Mg/(Mg+Fetot.); molar ratio (alumina saturation index) A/CNK = Al2O3/(CaO+Na2O+K2O); Modified Alkali-Lime Index MALI =
Na2O + K2O – CaO (from wt%, Frost et al., 2001)
46
Based on normative composition (CIPW), they correspond to tonalite, granodiorite,
and monzogranite, with small amounts of syenogranite and alkali-feldspar granite.
Except from two samples of metaluminous tonalite (sample I-10) and granite
(P160) and one strongly peraluminous granodiorite (P8) with A/CNK = 1.245, all
other orthogneisses have weak-to-moderate peraluminous composition with A/CNK
1.01–1.12. Almost all samples correspond to calc-alkaline rocks. The only exception
is the tonalite gneiss (I-10) with unusually high Fetot (10.42 wt % Fe2O3) and low K2O
(0.5 wt %).
Some geochemical characteristics of the orthogneisses are shown on a
spiderdiagram (Fig. 3) and on two element-element variation diagrams proposed by
Pearce et al. (1984) for discrimination of tectonomagmatic position of granitic rocks
(Fig. 4). Most orthogneisses have high large-ion lithophile elements (LILE) and Th
(Fig. 3) and display high LILE/HFSE elemental ratios. Only the tonalitic gneiss (I-10)
has low LILE (K, Rb, and Ba) but significantly higher Zr and Hf contents, which are
close to those in ―ocean-ridge granites‖ (ORGs) (Fig. 3). Geochemical features of this
sample are generally similar to some dacitic volcanic rock from the Cambrian
Volcano-Sedimentary Unit (Lower Cambrian dacite porphyry JR95-G47 from
Zarigan Mountain; Ramezani and Tucker, 2003), however, it is more evolved and thus
has higher SiO2 and lower CaO.
Amphibolites
Mafic rocks associated with orthogneisses are generally low in SiO2 (46.2 to 51.6
wt %), high in total Fe as Fe2O3 (8.6 – 13.5 wt %) and have variable contents of MgO
(12.2 – 2.5 wt %), Na2O (1.6 – 2.8 wt %) and K2O (0.2 – 1.7 wt %). They correspond
to subalkaline basalts or gabbros of tholeiitic affinities. The high MgO and mg-value
of sample P182 suggests a primitive composition. However, this rock contain gedrite
and kyanite and it is rich in Al2O3 (18.36 wt %). Samples P182 and I-5 have low
TiO2, P2O5 and incompatible elements including rare earth elements (Fig. 5) that could
result from accumulation of early magmatic phases (calcic plagioclase, olivine,
orthopyroxene). In contrast, sample P74 shows evolved basaltic composition with
high total Fe but with a very low mg-number and increased contents of incompatible
elements. Geochemical features of amphibolite samples are illustrated in a
spiderdiagram (Fig. 5) and selected widely used tectonic discrimination plots (Fig. 6).
47
They show high Th/Ta ratios and plot both in different fields of within plate, MORB
and volcanic arc basalts. Their geotectonic interpretation is discussed later.
Fig. 3. Spidergram of selected elemental abundances in orthogneisses normalized with the “ocean-
ridge granite” (ORG). Normalizing values are from Pearce et al. 1984).
Fig. 4. Position of the studied orthogneisses in selected discrimination diagrams for the tectonic
setting of granitoid rocks according to Pearce et al. (1984).
48
Fig. 5. Spidergram of MORB-normalized abundances of selected trace elements after Pearce
(1983) for amphibolites associated with orthogneisses from the Shotur Kuh metamorphic complex.
Fig. 6. Selected discrimination plots for the tectonic setting of analyzed amphibolites. a – Zr/4 -
2Nb - Y (Meschede 1986); b – Th - Hf/3 - Ta (Wood 1980). AI – within-plate alkali basalts; AII –
within-plate alkali basalts and within-plate tholeiites; B – E-type MORB; C – within-plate
tholeiites and volcanic-arc basalts; D – N-type MORB and volcanic-arc basalts.
49
Table 2. Selected microprobe analyses of minerals used for PT calculations
Garnet
------------------------------------------------------------------------------
Biotite Chlorite
-------------------------- ----------
Amphibole
-------------------------
Plagioclase
------------------------------------------------------------
Sample I-12 P95 P99 P34 P200 I-10a I-10b I-12 P95 P99 P200 I-10a I-10b P34 I-12 P95 P99 P34 I-10a I-10b
Rock
Orthogneiss
------------------------------------------------
Mic
--------
Amphibolite
-------------------
Orthogneiss
---------------------------
Mic
--------
Amphibole
------------------
Orthogneiss
----------------------------------------------
Amphibolite
------------------
c r
SiO2 37.37 37.97 36.67 37.89 37.54 36.72 38.49 38.19 36.58 37.19 36.70 24.11 41.43 42.39 39.50 63.71 62.87 60.65 62.3 61.81 63.42
TiO2 0.07 0.04 0.03 0.00 0.11 0.03 0.12 0.00 2.03 2.71 2.43 0.06 0.72 0.73 1.15
Al2O3 21.10 21.38 20.48 20.85 20.97 20.92 20.75 21.38 17.63 19.10 17.79 21.69 14.68 13.50 15.22 22.82 23.05 24.85 23.68 24.59 22.98
Fe2O3 0.75 0.00 0.94 0.73 0.00 0.00 2.16 0.03 2.38 0.00 0.01 0.14 0.00 0.04 0.12 0.00
FeO 32.84 32.35 32.82 31.89 28.15 32.15 21.48 26.73 17.87 16.37 18.12 25.79 14.38 21.03 17.62 0.00 0.00 0.00 0.00 0.00 0.00
MnO 2.69 2.25 1.69 1.54 1.54 0.76 3.84 0.47 0.00 0.04 0.00 0.16 0.16 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
MgO 3.84 3.89 3.81 3.86 2.63 2.48 2.10 1.31 11.61 12.03 11.16 15.11 8.65 5.51 8.24 0.00 0.00 0.00 0.00 0.00 0.00
CaO 2.21 2.61 2.58 3.27 8.61 6.36 11.41 12.22 0.02 0.34 0.00 0.06 11.34 11.09 11.30 3.94 4.93 6.44 5.04 6.37 4.22
Na2O 0.07 0.00 0.01 0.07 0.11 0.01 0.43 0.00 0.11 0.00 0.12 0.03 1.79 1.78 2.26 9.35 8.74 7.86 8.82 7.66 9.31
K2O 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 9.75 9.10 9.73 0.03 1.32 1.13 0.70 0.11 0.16 0.18 0.00 0.14 0.00
Total 100.90 100.5 99.03 100.1 99.75 99.43 100.8 100.3 95.59 96.88 96.05 87.05 96.85 97.17 96.00 99.93 99.89 99.99 99.86 100.7 99.93
Si 2.942 3.022 2.948 3.032 2.993 2.967 3.023 3.029 2.755 2.726 2.754 2.209 6.254 6.480 6.081 2.811 2.785 2.700 2.763 2.723 2.805
Ti 0.004 0.003 0.002 0.000 0.007 0.002 0.007 0.000 0.115 0.149 0.137 0.004 0.082 0.084 0.133 0.00 0.00 0.000 0.000 0.00 0.000
Al 1.958 2.006 1.940 1.966 1.959 1.993 1.921 1.998 1.565 1.650 1.573 2.342 2.612 2.433 2.762 1.186 1.204 1.304 1.238 1.277 1.195
Fe3+ 0.044 0.000 0.057 0.044 0.074 0.128 0.002 0.270 0.059 0.000 0.000 0.005 0.000 0.000 0.004 0.000
Fe2+ 2.162 2.009 2.206 2.134 1.792 2.172 1.411 1.774 1.126 1.004 1.137 1.976 1.815 2.688 2.269 0.00 0.00 0.000 0.001 0.00 0.000
Mn 0.179 2.154 0.115 0.105 0.103 0.052 0.255 0.032 0.000 0.002 0.000 0.012 0.020 0.000 0.000 0.00 0.00 0.000 0.000 0.00 0.000
Mg 0.451 0.222 0.457 0.460 0.311 0.551 0.246 0.156 1.303 1.315 1.248 2.064 1.947 1.256 1.892 0.00 0.00 0.000 0.000 0.00 0.000
Ca 0.186 0.461 0.222 0.280 0.731 0.299 0.960 1.039 0.002 0.027 0.000 0.006 1.834 1.816 1.864 0.186 0.234 0.307 0.239 0.301 0.200
Na 0.011 0.152 0.001 0.010 0.017 0.065 0.000 0.016 0.000 0.019 0.000 0.524 0.527 0.674 0.800 0.751 0.679 0.758 0.654 0.798
K 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.937 0.851 0.932 0.003 0.254 0.220 0.137 0.006 0.009 0.010 0.000 0.008 0.000
alm 0.726 0.414 0.735 0.714 0.610 0.707 0.491 0.591 An 0.188 0.236 0.308 0.240 0.312 0.200
py 0.151 0.046 0.152 0.154 0.105 0.179 0.086 0.052 Ab 0.806 0.755 0.681 0.760 0.679 0.800
grs 0.063 0.095 0.074 0.094 0.248 0.097 0.334 0.346 Or 0.006 0.009 0.011 0.000 0.008 0.000
sps 0.060 0.444 0.038 0.035 0.035 0.017 0.089 0.011
XMg 0.17 0.10 0.172 0.177 0.15 0.20 0.15 0.08 0.54 0.57 0.52 0.51 0.52 0.32
*-micaschist
50
2.6. Mineral chemistry
Orthogneisses
Garnet
Garnet from muscovite-free varieties is mostly rich in Fe and Ca (Alm59-66, Grs16-
18, Py15-18, Sps1-7). The amphibole- or epidote-bearing varieties have garnets with
relatively low Mg and higher Ca contents (Fig. 7). The most Ca-rich garnet comes
from the orthogneiss containing allanite rimmed by epidote (sample I8b). Most
garnets show weak but clear prograde zoning characterized by the decrease of Mn and
XFe = Fe2+
/(Mg+Fe2+
) from the core toward the rim. In some cases, garnet may show a
retrograde zoning (an increase of Mn and XFe) at the outermost rim part of the grains
(Fig. 8). The variation of Ca differs from sample to sample but mostly shows an
increase from core to rim. Garnet in the muscovite-bearing variety, including that with
microcrystalline texture, has relatively high Fe (Alm69-73) and Mg (Py14-20) and low Ca
(Grs5-9) abundances. Similar to garnet from Ms-free rocks, slight zoning from core to
rim can be observed in some grains. The spessartine content ranges between 3and 8
mol %.
Fig. 7. Composition of garnet from orthogneisses, amphibolites, and micaschists in the Shotur Kuh
complex. ´m´ and ´am´ are muscovite- and amphibole-bearing varieties, respectively.
51
Micas
Biotite from muscovite-free samples has lower XMg = 0.26–0.36 compared with
that in the muscovite-bearing variety (XMg = 0.51–0.55), but the two varieties have
similar Al (1.6–1.7 atoms per formula unit (a.p.f.u)) and Ti (0.1–0.2 a.p.f.u.) contents.
Biotite associated with chlorite, muscovite, and albite in mylonitized orthogneiss has
XMg = 0.32–0.34, which is similar to that in the muscovite-free orthogneisses, but with
lower Al (1.4 a.p.f.u.) content. Muscovite has low Si (3.0–3.1 a.p.f.u.) with relatively
high paragonite content (10–20 mol %), and only the microcrystalline variety of
orthogneiss has low (3 mol %) paragonite content.
Plagioclase
Plagioclase in the muscovite-free varieties has a high anorthite content (An22-31
mol %), and some higher An contents come from the cores of large plagioclase grains.
The coarse-grained, muscovite-free variety has An18-24, while that from the
microcrystalline variety has relatively high Ca content (An23-30).
Other minerals
Epidote was analysed in muscovite-free orthogneisses, and it has XAl (Al/(Al + Fe3+
))
= 0.75–0.86.
Fig. 8. Compositional profiles of
garnet from muscovite-bearing
orthogneiss (a, sample P99) and
from micaschist (b, sample P200).
52
Amphibolite
Garnet
Garnet from amphibolite is mostly almandine-grossular solid solution. Its
composition varies from sample to sample between Alm49-64, Grs31-34, Py3-9 and Sp1-9.
Garnet from biotite-amphibole gneiss (sample P34) has higher Mg and lower Ca
(Alm56-63, Grs21-25, Py10-13, Sps2-9) and it is mostly homogeneous, although slight
zoning characterized by an increase in Mg and Fe and a decrease in Mn and Ca
toward the rim can be seen.
Amphiboles
Amphibole in kyanite-free amphibolites is mostly pargasite and ferropargsite, with
Si = 6.2– 6.4 and XMg = 0.53–0.30. Ferropargasite is present in sample I-10a, along
with relatively high-Fe garnet (Fig. 7). The A-site of amphibole is occupied by 0.50–
0.65 atoms, and NaM4
position is in the range of 0.25–0.45 a.p.f.u. Amphibole from
biotite-bearing amphibolite has XMg = 0.38–0.45 with NaM4
about 0.2, and Mg-rich
tschermakite with XMg = 0.9 (NaM4
= 0.40–0.44 a.p.f.u., Si = 6.3 a.p.f.u.) is associated
with gedrite in kyanite-bearing amphibolite (sample P182). The coexisting gedrite,
which has XMg = 0.7, is rich in Al (Al2O3 = 17.3 wt.%) and Na (Na2O = 2.3 wt %), and
the A-site occupancy is about 0.4 a.p.f.u. Considering the coupled substitution of
tschermakite (Ts): AlVI
+ AlIV
= MgVI
+ SiIV
and edenite (Ed): NaA + AlIV
= A +SiIV
(Robinson et al., 1971), the analysed orthoamphibole shows Ed/Ts = 0.33 and has a
composition close to ideal gedrite.
The analysed gedrite in the kyanite-bearing amphibolite has Al = 2.9 a.p.f.u. and
Na = 0.55 a.p.f.u., which are slightly lower compared with typical literature data
(Spear, 1980). Robinson et al. (1971) emphasized the importance of Na in the A site
of gedrite and postulated an ideal end-member gedrite composition of Na0.5(Mg,Fe2+
)2
(Mg,Fe2+
)3.5Al1.5Si6Al2O22(OH)2. This formula represents a combination of the
edenite and tschermak‘s substitutions in a ratio of one to three. The analysed gedrite
has lower A-site occupancies and AlVI compared with ideal gedrite, with A = 0.5 and
AlVI
= 2.0 (Spear, 1980).
53
Plagioclase
Plagioclase composition in amphibolite is An30-31 in sample I-10a and An18-20 in
sample I-10b. Oligoclase-andesine with 28–30 mole % of An occurs in the biotite-
bearing amphibolite, and more Ca-rich andesine with An33-35 is present in the kyanite-
bearing amphibolite (sample P182).
Micas
Biotite from the kyanite-bearing amphibolite (P182) has XMg = 0.7. The phengite
analysed in this sample has a high Si content (Si= 3.36–3.40 a.p.f.u).
Micaschist
Garnet (sample P200) is rich in almandine and shows strong zoning (Fig. 8) with a
Mn-rich core (Alm49,Grs16,Py3,Sps31), a progressive increase in Mg and a decrease in
Mn and XFe (Alm70,Grs18,Py10,Sps1) towards the rim. Biotite has XMg =0.52–0.55, the
silica and paragonite contents in muscovite correspond to 3.0 a.p.u. and 11–16 mol %,
respectively. Plagioclase is pure albite.
2.7. PT conditions of metamorphism
Most of the studied rocks have simple mineral assemblages containing feldspars,
biotite ± garnet in orthogneisses and amphibole ± garnet in amphibolite. Plagioclase is
not always totally equilibrated and may preserved its igneous composition in the cores
of large porphyroclasts. In one case, relics of clinopyroxene were also observed in
amphibolite. To constrain the PT conditions of metamorphism, well-foliated and
recrystallized samples of orthogneisses and amphibolites were selected. Temperature
was calculated using the exchange thermometers of garnet-biotite (Ganguly et al.,
1996; Holdaway, 2000), garnet-muscovite (Hynes and Forest, 1988), and amphibole-
garnet (Graham and Powell, 1984; Perchuk and Lavrentjeva, 1983; Dale et al., 2000).
Because of the lack of Al-silicate in garnet-bearing gneisses, pressure was estimated
using the garnet-biotite-plagioclase-quartz barometer of Wu et al. (2006) and the
plagioclase-garnet-amphibole-quartz of Kohn and Spear (1990) in combination with
the occurrence of kyanite in amphibolite. Possible plausibility of the calculated PT
conditions was tested by the pseudosection method, using the Perplex 07 software
(Connolly, 2005: version 07) with the internally consistent thermodynamic data set of
54
Holland & Powell (1998: 2003 upgrade) for muscovite-bearing orthogneiss (P99).
The solution models of minerals that were used are plagioclase (Newton et al., 1980)
biotite, chlorite, and garnet (Holland and Powell, 2003).
Three orthogneiss samples (I-12, P95 and P99) and one micashist sample (P200)
were selected for garnet-biotite thermometry, and the temperatures obtained using
three calibrations for the orthogneisses are in the range of 577–645 °C (Table 3) but
show a slight increase from the calibration of Ganguly (1996) through Holdaway
(2000) to Wu and Cheng (2006). Relatively higher temperatures (615–695 °C) were
obtained for the garnet-bearing the amphibolite and amphibole-garnet-bearing
orthogneiss using garnet-amphibole thermometry with the calibration of Graham and
Powell (1984) and Dale et al. (2000). The calibration of Perchuk and Lavrentjeva
(1983) gave about 70 °C lower temperatures for these samples.
Fig. 9. Pseudosection (MnNCKFMASH system) constrained for garnet-muscovite-bearing gneiss
(sample P99) from the Shotur Kuh complex. All fields contain excess quartz. The open and filled circles
indicate intersections of isopleths (XFe, XCa in squares and XMn in ellipses) for core and rim
compositions of garnet.
55
Table 3. Results of exchange thermobaromery for gneisses and amphibolite
---------------------------------------------------------------------------------------------------------
Gneisses Grt-Bt: T (oC) Grt-Bt-Pl-Qtz: P (Kbar)
sample Ganguly Holdaway Wu Wu
I-12 577 ± 6 608 ± 7 631 ± 5 7.45±0.14
p95 596 ± 42 610 ± 58 629 ± 20 6.82±1.53
p99 602 ± 46 645 ± 75 633 ± 16 7.62±0.71
p200 541 ± 42 544 ± 46 578 ± 37
Amphibolite Grt-Hbl: T (oC) Grt-Hbl-Pl-Qtz: P (Kbar)
sample GP PL DHP KS DHP
I-10a 660±35 530±54 615±21 10.1±0.5 9.8±0.1
I-10b 684±26 558±16 680±14 11.±0.4 11.8±0.5
p34 668±48 601±49 695±24 9.9±0.3 10.2±0.4
---------------------------------------------------------------------------------------------------------
Ganguly-Ganguly et al. (1996), Holdaway-Holdaway (2000), Wu-Wu et al. (2006), GP-Graham and Powell
(1984), PL-Perchuk and Lavrantjeva (1983), DHP-Dale et al. (2000). KS -Kohn and Spear (1990). Grt-garnet,
Bt-biotite, Pl-plagioclase, Hb-Hornblende, Qtz-quartz,
Pressures obtained by the exchange barometry of Wu and Cheng (2006) for the
orthogneisses were in the range of 6.8–7.6 kbar, but the garnet-plagioclase-
amphibole-quartz barometry in amphibolite gave higher pressures of 10–12 kbar
using the calibration of Kohn and Spear (1990) and Dale et al. (2000). The higher
pressure for sample I-12b is due to its more sodic plagioclase composition (An18)
compared with the other samples that have plagioclase with An29-31. Similar pressures
(10± ± garnet-amphbole-plagioclase-equilibria were
obtained using average PT (Thermocalc, version 2.7) and the thermodynamic data set
of Holland and Powell (1998).
The results of pseudosection with isopleths of XCa, XFe and XMn in garnet for
sample P99 indicate 7.2 kbar/610 °C for core compositions and 8.4 kbar/660 °C for
rim compositions of garnet (Fig. 9). These PT conditions are confirmed by all three
isopleths of XCa, XFe, and XMn that intersect one another in the four variant fields with
garnet, muscovite, biotite, and plagioclase. This temperature is about 20 °C higher
than the average temperatures obtained by all exchange thermometers, but it is within
the error range resulting from analytical measurements and calculations. Pressure
conditions obtained from the pseudosection fit well with those derived from the
exchange barometry in orthogneisses. Such pressures are confirmed also by the
presence of kyanite in the Al-rich metabasite (sample P182), which, according to data
from the literaure (e.g., Cooper, 1980; Ward, 1984; Spear, 1982), indicates a pressure
greater than 6 kbar. For amphibolite-facies temperatures, Selverstone et al. (1984)
56
suggested that kyanite reflects pressures higher than those appropriate for the stability
of the common amphibolite assemblage. Experimental studies corroborate this idea,
and it was predicted (Schreyer and Seifert, 1969) that Ged + Ky should be stabilized
at pressures of about 10 kbar and that they are a precursor of Tlc + Ky "whiteschists"
(Schreyer, 1973).
The phyllitic-micaschist (sample P200) gave temperatures of 541–578 °C (Table
3), which are 50–60 °C lower than those in the orthogneisses. Pressure conditions
were not estimated, but they seem to be close to that in the orthogneisses.
2.8. Geochronology
The age of igneous crystallization of the Shotur Kuh orthogneiss-amphibolite
complex was obtained by laser ablation ICPMS U-Pb dating of zircon extracted from
an orthogneiss sample of granodiorite composition (P160). The data constrain the
crystallization age of the granodiorite to be 547 ± 7 Ma (2 sigma Concordia age; Fig.
10, Table 4). A similar age of 566 ± 31 Ma (n = 8, MSWD=1.3) has been recently
obtained for the Shotur Kuh Complex orthogneisses using ion microprobe and
thermal-ionization zircon dating (Hassanzadeh et al., 2008).
Fig. 10.Pb concordia diagram for zircon from orthogneiss (sample 160) in Shotur Kuh complex.
57
Both ages are consistent with the U-Pb zircon ages of granitic and volcanic rocks
reported from several localities in Central Iran, including Saghand area (Ramezani
and Tucker, 2003) and Sanandaj– Sirjan region (Hassanzadeh et al., 2008).
Ar-Ar age dating was performed on metamorphic muscovite from a metagranite
sample showing well–preserved, equilibrated fabric in amphibolite facies conditions.
However, a distinct low-temperature alteration is clearly visible and caused severe
alteration of biotite and also minor alteration of feldspars. The age pattern of the
muscovite from this sample corresponds well with the crystallization sequence
observed in thin section. It shows a distinct saddle-shaped pattern that points to a
moderate age rejuvenation due to diffusional loss of radiogenic Ar during low-
temperature reheating (Fig. 11). A single sample may not be sufficient to make far-
reaching conclusions, but from our experience we conclude that the mica flakes
contain age domains that are older than 166 Ma (probably about 170–180 Ma), which
reflects the original cooling after metamorphism. A Lower Jurassic age of 171.8 ± 2.7
was also obtained by K-Ar dating of muscovite (sample I-13), although biotite from
the same sample gave 208 Ma (Table 5). A younger age of 141 Ma was obtained for
biotite in sample I-12.
Fig. 11. Ar/Ar ages of 166 Ma for muscovite in micaschist (sample P98) from the Shotur Kuh complex
(Central Iran).
58
Table 4. U-Pb and Pb-Pb Laser Ablation ICPMS data for zircons from sample P160
Analysis Isotopic ratios Ages Ma
no. 207
Pb/235
U ± 1 sigma 206
Pb/238
U ± 1 sigma 207
Pb/206
Pb ± 1 sigma 207
Pb/235
U ± 1 sigma 206
Pb/238
U ± 1 sigma 207
Pb/206
Pb ± 1 sigma
1 0.6685 0.0294 0.0839 0.0026 0.0578 0.0007 519.8 22.8 519.1 16.0 522.6 25.8
2 0.7143 0.0315 0.0889 0.0032 0.0583 0.0007 547.3 24.1 549.0 19.5 540.2 27.8
3 0.6824 0.0284 0.0920 0.0024 0.0538 0.0012 528.2 22.0 567.6 14.8 361.6 49.4
4 0.6750 0.0243 0.0838 0.0025 0.0584 0.0006 523.8 18.8 518.7 15.4 545.9 21.7
5 0.7023 0.0181 0.0904 0.0015 0.0564 0.0005 540.1 13.9 557.6 9.5 467.1 20.4
6 0.6947 0.0343 0.0848 0.0027 0.0594 0.0010 535.6 26.5 524.9 16.5 581.5 36.3
7 0.7496 0.0460 0.0936 0.0046 0.0581 0.0009 568.0 34.9 576.9 28.2 532.5 32.4
8 0.6739 0.0348 0.0883 0.0025 0.0554 0.0015 523.1 27.0 545.3 15.3 427.4 61.9
9 0.7252 0.0474 0.0935 0.0030 0.0562 0.0009 553.7 36.2 576.4 18.7 461.5 37.3
10 0.7203 0.0445 0.0897 0.0035 0.0582 0.0012 550.9 34.0 553.9 21.7 538.2 43.3
11 0.7500 0.0297 0.0936 0.0026 0.0581 0.0009 568.2 22.5 576.5 16.2 535.3 34.8
12 0.7280 0.0285 0.0887 0.0022 0.0596 0.0009 555.4 21.8 547.6 13.7 587.5 33.4
13 0.7397 0.0283 0.0892 0.0031 0.0601 0.0006 562.2 21.5 550.9 18.9 608.2 22.3
Table 5. Results of K-Ar dating from muscovite and biotite in the Shotur Kuh metamorphic complex
Samples Rock type Mineral K2O (wt.%)
% rad. 40
Ar
tot. 40
Ar
rad.40
Ar
(10-11
mol/g) Age (± 2σ) in Ma
I- 12 orthogneiss biotite 8.446 93.3 178.6 141.2 ± 2.2
I- 13 orthogneiss biotite 8.383 90.3 266.9 208.7 ± 3.2
I- 13 orthogneiss muscovite 8.454 96.4 219.4 171.8 ± 2.7
59
2.9. Discussion
Evidence for a Pre-Cambrian to Cambrian magmatic arc
The new geochronological data for crystallization ages of granitoid protoliths of
orthogneisses from the SKMC are quite comparable with the granitoid ages reported
by Ramezani and Tucker (2003) from the Saghand area in East-Central Iran who
interpreted the igneous rocks as a magmatic arc of Late Neoproterozoic to Early
Cambrian age (Fig. 12). This interpretation was recently confirmed by
geochronological data from granites and orthogneisses from central Iran
(Hassanzadeh et al., 2008). These ages, which range from Late Neoproterozoic to
Early Cambrian, match the mostly juvenile Arabian–Nubian shield and Peri-
Gondwanan terranes that have formed after the main phase of Pan-African Orogeny.
Fig.12. The “Modified Alkali-Lime Index” (MALI) vs SiO2 diagram after Frost et al. (2008) for Shotur
Kuh Metamorphic Complex orthogneisses (field labelled SKMC, circles represent individual analyses).
Other fields represent: BSG - Boneh Shurow granitic gneisses from Central Iran (Ramezani and
Tucker, 2003), CGS - Cambrian Granitic Complex from Central Iran (Ramezani and Tucker, 2003),
BM- granitoids of the Bitlis Massif in SE Turkey (Ustaömer et al., 2008). For comparison, variation
fields for typical magmatic arc intrusive complexes of the Peninsular Ranges and Tuolomne Batholith
are displayed (from Foster et al., 2008). Terminology for individual igneous suites: a – alkalic, a-c –
alkali-calcic, c-a – calc-alkalic, c – calcic.
Consistent with paleogeographic reconstructions for the Late Neoproterozoic (Powell,
1980; Laver and Scotes, 1987; De Wit et al., 1988; Unrug, 1997) the arc was formed
by the closure of the Chapedony Ocean (Proto- Tethys, Fig. 13) (Ramezani and
60
Tucker, 2003). Following the Pan-African Orogeny and the consolidation of the
basement, the Precambrian craton of Iran, Pakistan, central Afghanistan, southeastern
Turkey and Arabia became a relatively stable continental platform with epicontinental
shelf deposits (mainly clastics) and was characterized by a lack of major magmatism
or folding (Nadimi, 2007).
Fig.13. Schematic Gondwanaland reconstruction in Early Cambrian based on Molleweide projection
of tectonic plates at 540 Ma by the PLATES Project (1999), with modifications by Remezani and
Tucker (2003). Precambrian cratons are: Af- African, Am- South American, An- Antarctic, Au-
Australian and In- Indian plates, Ch-South China, L-Lhasa and MT- Menderes-Tauruns Blocks. Stars
refer to Peri-Gonwana arc plutons.
Despite large compositional variability, the overall geochemistry of the
orthogneiss varieties from the Shotur Kuh complex is compatible with an origin of
their granitoid protoliths within a magmatic arc. Based on the trace-element
discrimination diagrams (Pearce et al. 1984), the majority of orthogneisses plot within
the ―volcanic arc granite‖ field (Fig. 4). We suggest that the arc was situated at a
continental margin where melting and recycling of abundant older continental
material could play a significant role. Some orthogneiss analyses are very comparable
with rocks described by Ramezani and Tucker (2003) as being members of the Late
Neoproterozoic to Early Cambrian arc igneous assemblage from the Saghand area
(see Fig. 1 and Table 6)—namely, with the ―Cambrian Granitoid Suite‖ (CGS) and
with the acidic orthogneiss ―Boneh-Shurow Granitic Gneiss‖ (BSG) dated as Late
Neoproterozoic. When comparing these orthogneisses with Late Neoproterozoic to
Early Cambrian igneous and metaigneous rocks in the nearby regions they show close
61
similarity to continental arc-related granitoids and metagranitoids in SE Turkey
(Ustaömer et al., 2009). Fig. 12 displays geochemical signatures of the Shotur Kuh
orthogneisses in the Frost et al. (2001) diagram and compare them with some plutonic
rocks from other parts of the peri-Gondwanan and also much younger arc related
plutonic complexes from North America. Despite of their rather broad scatter, the
Shotur-Kuh orthogneisses fit well with arc-related calcic and calc-alkalic suites and
together with other granitoid or metagranitoid complexes from the peri-Gondwanan
arc they correspond to silica-rich, evolved rocks. Although some geochemical
features, namely the high total Fe and Zr contents, of the tonalitic orthogneiss (sample
I-10) appear to be far from typical arc assemblages they are broadly similar to some
subvolcanic members of the Cambrian Volcano- Sedimentary Unit of Saghand, which
has been interpreted as arc-related (Ramezani and Tucker, 2003).
Table 6 Comparison of selected geochemical parameters for the studied orthogneisses
with other Late Neoproterozoic to Cambrian granitoids in Central Iran and SE Turkey
Orthogneisses,
Shotur Kuh Metam.
Complex (this study)
Cambrian Granitoid
Suite (Ramezani &
Tucker 2003)
Boneh-Shurow
Granitic Gneisses
(Ramezani & Tucker
2003)
Bitlis Massif, SE
Turkey (Ustaömer et
al. 2008)
SiO2 (63.72) 64.3 - 75.9 65.9 - 76.3 71.7 – 77.7 68.0-78.3
K2O (0.47) 1.33 - 4.61 2.86 - 4.92 2.7 – 4.8 0.2 – 4.6
K2O/Na2O (0.146) 0.39 - 1.6 0,8 - 1,39 0.6 – 1.3 0.02 – 1.2
MALI (-2.0) 1.6 – 6.8 2.9 – 8.7 5.8 – 7.2 1.6 – 7.8
A/CNK (0.833) 0.866 -1.24 0.923 - 1.004 0.94 – 0.99 0.82 – 1.09
Th/Ta 9.9 - 29.3 17.5 - 44.4 11 - 22 -
Eu/Eu* 0.20 - 1.02 0.38 - 0.59 0.30 – 0.68 -
Values in parentheses represent the orthogneiss I-10 that is significantly different from other rock
analyses representing the Shotur Kuh metamorphic complex. MALI is “Modified Alkali-Lime Index”
Na2O + K2O – CaO (Frost and Frost, 2001); A/CNK is molar ratio Al2O3/(CaO + Na2O + K2O).
The tectonic positions of mafic rocks (amphibolites) spatially associated with
orthogneisses, are difficult to interpret, mainly because of metamorphism and
deformation, which could have modified their original geological relationships. Some
small amphibolite bodies could represent original mafic enclaves. However, the
majority of mafic bodies may represent metamorphosed primary dykes.
The geochemical indications for the original tectonic setting of mafic rocks are
rather ambiguous. In various discrimination diagrams, amphibolites plot in the fields
of ―within-plate basalts‖and MORBs, and also of volcanic arc basalts. Because of the
relatively high Th contents, most samples are shifted toward the ―Calc-alkaline
basalt‖ field or plot within this field (Fig. 6). The increased Th/Ta ratios (Fig. 5) may
62
indicate that the original basaltic magmas were derived from a mantle source affected
by subduction-related processes.
Some geochemical features of amphibolites are consistent with the origin of their
protolith magmas at a destructive plate margin – i.e., in a magmatic arc (increased
LILE/HFSE and Th/Ta element ratios, Fig. 5). However, the ―subduction fingerprint‖
is rather weak and could also be explained in terms of partial melting of the sub-
continental lithospheric mantle (with the long term ―subduction memory‖) and/or
contamination of mafic magmas by continental crust (including protoliths of
orthogneisses). Except for the two most mafic samples that are probably affected by
the accumulation of early mineral phases, the rocks display relatively high TiO2
content, which is in contrast to the composition of typical volcanic arc magmas. Mafic
to intermediate rocks with strong, typical fingerprints of subduction-related magmas
are absent, and therefore the arc igneous assemblage is incomplete and the
outcropping rock complexes involving metamorphosed mafic dykes may represent an
extensional setting of the continental back-arc rather than a true arc near the volcanic
front. However, even such a setting can be supposed to be a part of a much greater
Late Neoproterozoic-Early Paleozoic orogenic system that was active along the Proto-
Tethyan margin of the Gondwanaland supercontinent, extending at least from its
Arabian margin to the Himalayan margin of the Indian subcontinent (Ramezani and
Tucker, 2003).
Metamorphic evolution
Mineral assemblages observed in the studied rocks indicate a medium-pressure,
barrovian-type metamorphism for which a maximum pressure of c. 8 kbar and a
temperature of about 650°C were obtained. This metamorphism produced garnet,
biotite, plagioclase, quartz, and muscovite in orthogneisses, and garnet, amphibole,
plagioclase, and quartz in amphibolite. The overlaying Pre-middle Triassic
sedimentary sequences were also affected by this metamorphism, but they show a
lower temperature (550 °C). Garnet, both in metaigneous and metapelitic rocks,
indicates prograde zoning from cores to rims of grains. This zoning is strong in
metapelites and weak in metaigneous rocks, where it shows additional modification at
rims (retrograde zoning) that probably occurred during cooling. The weak prograde
zoning in metaigneous rocks could be the result of either higher temperatures or
longer relaxation times at high temperature. Lithological similarity but different PT
63
conditions between Pre-middle Triassic pelitic rocks and garnet micaschists suggest
either tectonic juxtaposition of single unit with differing metamorphic grade or the
micaschists belong to an older basement unit, which reached PT conditions close or
similar to the orthogneisses.
The limitation of the Middle- to Lower-Jurassic ages of 165 Ma for the
amphibolite facies metamorphism, obtained by Ar-Ar data on muscovite, is supported
by the presence of the Upper Triassic-Lower Jurassic Shemshak formation and by the
Middle-Jurassic pasammopelitic rocks with conglomerates containing pebbles of the
basement orthogneisses and amphibolite. The stratigraphically well-defined
Shemshak formation in Central Iran (e.g., Assereto, 1966; Stampfli, 1978) shows
lower greenschist facies metamorphism with fabrics comparable to those in the
underlying limestone-dolomite formation (Rahmati-Ilkhchi, 2008). Because the
Middle- Jurassic sediments with conglomerates show only very-low-grade
metamorphic conditions, the greenschist facies overprint (fine-grained white mica,
chlorite with albite, epidote, and, locally, also brown-green biotite replacing garnet or
plagioclase) in the basement rocks could be related to retrogression during
exhumation.
The results of our petrological and geochronological study agree well with the
geodynamic scenario for closure of the Neotethyan basin in the southwestern part of
Iran (e.g., Stöcklin, 1968; Barberian and King, 1981; Sengor, 1991; Glennie, 2000;
Sheikholeslami et al., 2008; Ghasemi and Talbot, 2006). This basin was formed by
continental rifting, separating the Iranian and the Arabian plates (Fig. 14). Formation
of a magmatic arc with an accretionary prism during the Early Cimmerian Orogeny
(Sheikholeslami et al., 2008; Paul et al., 2003) is coeval with the Middle Jurassic
cooling age of white mica from the Shotur Kuh Complex, which defines the
collisional processes and metamorphism of the basement rocks of the Central Iran
Block.
The significance of the Shotur Kuh metamorphic complex for the geological history of
the Great Kavir Block
The investigated metaigneous rocks from the Shotur Kuh complex show arc-
related signatures similar to those described by Ramezani and Trucker (2003) and
confirm a uniform geotectonic position for the Central Iran micro-continent during the
Late Neoproterozoic, including the GKB. This interpretation is also supported by
64
comparative stratigraphy of Paleozoic to Mesozoic sedimentary sequences among
different crustal blocks (Stöcklin, 1968; Takin, 1972; Crawford, 1977; Berberian et
al., 1981).
Fig.14. Geodynamic reconstruction of southwest margin of the central Iran and north Gondwanian
margin from Late Paleozoic to Cenozoic (modified from Berberian and King, 1981, and Sheikholeslami
et al., 2008). a) Development of rift-type basin from Permian. (b) Subduction of the Neotethys oceanic
basin in Late Triassic (Early Cimmerian phase) and formation of an accretionary prism and magmatic
arc (granite/granodiorite of Chah Dozdan). (d) Collision and generation of an orogenic prism with
metamorphism and exhumation of the basement units (Saghand-Shotur Kuh metamorphic complexes)
and development of volcanic arc of Urumieh-Dokhtar zone. A-Arabian plate, Z-Zagros, Ss-Sanandaj-
Sirjan, CI-Central Iran and Al-Albroz
65
Most basement rocks in Central Iran are exposed along tectonic zones separating
individual blocks. In some cases, these zones are followed by Paleozoic to Cenozoic
ophiolites with or without blueschists (Fotoohi Rad et al., 2005; Bagheri and Stampfli,
2008), which suggests large-scale tectonic activity with rifting and subduction. The
Sabzevar zone with ophiolites and blueschists is the best example of a back-arc basin
that was opened during Cretaceous north from the Neothethys ocean (Bagheri and
Stampfli, 2008). The SKMC is exceptional, since it occurs in the central part of the
GKB. However, the positions of the SKMC and some mafic and ultramafic rocks near
Kuh Siah Poshteh (100 km from the Shotur Kuh complex) along the E-W- trending
Torud fault zone may suggest that the Torud fault zone was also active prior to
Tertiary volcanism. Considering the N-S compression of the Shotur Kuh complex
with south-vergent isoclinal fold axes (Rahmati- Ilkhchi et al., 2008), the Torud
tectonic zone may represent a large-scale thrust zone that was responsible for
formation (?) and exhumation of the amphibolite facies metamorphic rocks during the
Jurassic. The rapid increase in the degree of metamorphism from top to bottom in the
Pre-middle Triassic rocks in contact with orthogneiss is either due to tectonic
reduction of relatively thick sequences or the result of heating of partially exhumed
orthogneiss-amphibolite rocks to the upper-crustal level. To confirm or reject the
hypothesis of large thrust and underplate tectonics along this zone, more detailed
structural analyses and dating of mafic and ultramafic rocks is needed.
2.10. Conclusions
Geochemical analyses and U-Pb zircon age data from orthogneisses in the Shotur Kuh
metamorphic complex indicate that these rocks are derived from granitoid protoliths
(granite, granodiorite, and tonalite) formed during Late Neoproterozoic arc-related
magmatism. This magmatism, which is known also from other zones and blocks in
Central Iran, confirms that the GKB, along with the rest of the blocks of the Central
Iran micro-continent, were part of the Neoproterozoic- Early Paleozoic orogenic
system that was active along the Proto-Tethyan margin of the Gondwanaland
supercontinent. In the Shotur Kuh complex, these rocks were affected by barrovian-
type metamorphism that reached amphibolite facies conditions in the deeper part of
the exposed basement with orthogneisses and amphibolites, and lower amphibolite
facies in the sedimentary cover. Both igneous and sedimentary rocks show evidence
66
of prograde metamorphism with subsequent cooling. The results of petrological
studies with structural data about N-S compression and north-dipping structures
suggest that the Lower- to Middle-Jurassic metamorphism and exhumation of the
Shotur Kuh complex occurred under conditions of south vergent thrust tectonics along
the E-W zone. The latter was probably related to the closure of the Neotethyan basin
and subsequent collision during the Early Cimmerian Orogeny.
67
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CHAPTER 3
Mid-Cimmerian, Early and Late Alpine orogenic events in
the Shotur Kuh metamorphic complex, Great Kavir block,
NE Iran
Mahmoud Rahmati-Ilkhchi1,2
, Petr Jeřábek1, Shah Wali Faryad
and Hemin A. Koyi
3
1 Institute of Petrology and Structural Geology, Charles University in Prague, Albertov 2, 128 43
Prague, Czech Republic
2 Geological Survey of Iran
3 Hans Ramberg Tectonic Laboratory, Institute of Earth Sciences, Uppsala University, Villava¨gen 16,
Uppsala S-752 36, Sweden
Abstract
The Shotur Kuh complex, exposed in the NE part of the Great Kavir block, is
composed of amphibolite facies metaigneous rocks and micaschist, and of lower-
grade Permian–Miocene cover sequences that experienced four main deformation
phases and at least two metamorphic events. The D1 deformation phase is associated
with a prograde metamorphism that in the basement reached amphibolite facies
conditions. This Barrovian type metamorphism with field gradient of 20–22 °C/km
was related to collision-induced crustal thickening. The D2 event corresponds to post-
collisional exhumational upflow of middle crust resulting in updoming of the
basement core and its top-to-the-Northwest unroofing along a low angle detachment
shear zone at the basement/cover boundary. The D1 and D2 events are considered as
Mid-Cimmerian in age as they affected also the Lower Jurassic Shemshak Formation
and are sealed by the Middle Jurassic conglomerates. The D3 folding event,
characterized by NE–SW shortening, affected also the Cretaceous limestones and it is
sealed by Paleocene conglomerates. Considering Late Cretaceous age of this
deformation, it is related to the Late-Cimmerian–Early Alpine orogeny that resulted
from the Cenozoic closure of the Neo-Tethys oceanic tract(s) and convergence
between Arabian and Eurasian plates. The D4 folding event, characterized by NW–SE
shortening, affected also the Miocene conglomerates implying its Late Miocene to
post-Miocene age. This deformation event is associated with Late Alpine Alborz and
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Zagros phase of convergence between Arabia and Eurasia during Late Cenozoic and
could be combined with a left-lateral activity along the Great Kavir fault bounding
system.
3.1. Introduction
The Cimmerian block in Central Iran (Stöcklin, 1968) is part of the Alpine-Himalayan
orogenic system in western Asia and consists of individual crustal segments that are
represented by Alborz, Sanandaj-Sirjan zone and Central Iranian domain (Fig. 1a). It
is formed by crystalline basement consolidated during the Neoproterozoic–Early
Cambrian Pan-African orogeny and represents former passive margin of Gondwana
that separated in Ordovician to Silurian and collided with Eurasia due to closure of the
Paleo-Tethys in Middle to Late Triassic Early-Cimmerian event (Stocklin, 1974;
Berberian and King, 1981; Sengör et al., 1988; Sengör, 1990; Stampfli et al., 1991;
Sengör and Natal‘in, 1996; Bagheri and Stampfli, 2008). In Jurassic–Cretaceous, the
ongoing subduction of Neo-Tethys in the south induced back arc spreading and partial
fragmentation of Cimmerian block, which again amalgamated with the closure of
Neo-Tethys in Upper Cretaceous to Eocene Late-Cimmerian–Early Alpine orogeny
(e.g. Ramezani and Tucker, 2003; Bagheri and Stampfli, 2008). Recent increase in
number of geochronological age data, obtained from the Cimmerian basement units,
have revealed also Jurassic (181–156 Ma) orogenic activity in the Sanandaj-Sirjan
zone (Khalatbari-Jafari, 2003; Rachidnejad-Omran et al., 2002; Sheikholeslami et al.,
2003, 2008; Falznia et al., 2007) and the Central Iranian domain (Bagheri and
Stampfli, 2008; Rahmati-Ilkhchi et al. in press). This activity has been attributed to a
series of back-arc extensional events and compressional cycles controlled by the
north-dipping Neo-Tethyan subduction zone (Bagheri and Stampfli, 2008) and/or to a
collisional interaction with Arabian plate prior to opening of the Neo-Tethys ocean
(Falznia et al., 2009). The Late Alpine event is represented by the late Miocene
Zagros and Alborz phase of continuous N–S convergence in the region controlled by
the movement of Arabian and Eurasian plates following the Neo-Tethys closure (e.g.
Alavi, 2004; Guest et al., 2006).
In this work, we further extend the evidence for Lower/Middle Jurassic orogenic
activity in the Central Iranian domain and present an overview of deformation
structures and related metamorphic record within the basement and para-
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autochthonous sedimentary cover of the Shotur Kuh metamorphic complex in the
northeastern part of the Great Kavir block. Spatial and temporal variations in the
deformation-metamorphic record within the Permian–Miocene cover are used to
relate the local deformation pattern to the main orogenic processes that affected the
Central Iranian domain.
3.2. Geological setting
The Central Iranian domain consists of several crustal blocks, from east to west the
Lut, Tabas, Yazd and Great Kavir (Fig. 1a), distinguished on the basis of lithological
correlation and main fault pattern (e.g. Stocklin, 1968; Takin, 1972; Crawford, 1977;
Berberian et al., 1981). Although most of the area is covered by thick sedimentary
sequences, which despite an occurrence of locally significant sedimentary facies
and/or thickness variations are largely comparable, the basement rocks are exposed
mostly along the block boundaries.
The Shotur Kuh complex exposed in the northeastern part of the Great Kavir
block between two parallel NE–SW trending Anjillo and Torud faults (Fig. 1b), is a
metamorphic complex with an E–W trending elliptical basement core domain (ca 20
km long and 11 km wide) and overlying para-autochthonous Permian–Miocene cover
sequences (Fig. 1c). The basement as well as the cover is intruded by a series of
Eocene volcanic rocks of dacite and andesite composition (Rahmati-Ilkhchi, 2002).
Most of the basement rocks are represented by orthogneiss with intercalations of
amphibolite and metagranitoids. The orthogneisses are derived from tonalite,
granodiorite, monzogranite, syenogranite and alkali-feldspar granite, and the
amphibolites represent former basaltic dyke swarms with different orientation
intruding the granitoid complex. The relative abundance of amphibolite ―dyke‖ bodies
in the western part of the basement vanishes towards the north and east. The
granitoids are derived from arc-related magmatism that was active during
Neoproterozoic–Early Cambrian (547±7 Ma; Rahmati-Ilkhchi et al, in press).
Small amounts of garnet-bearing micaschists are exposed at the contact between
basement orthogneisses and pre-Middle Triassic cover rocks in the south of the Shotur
Kuh complex.
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Fig.1. (a) Simplified tectonic map of eastern Iran showing constituent crustal blocks (compiled from
Ramezani and Tucker, 2003; Alireza Nadimi, 2006). (b) Schematic map of the Torud area showing
location of villages, mountains and important faults after Hushmandzadeh et al. (1978). (c) Simplified
geological map of the Rezveh area with location of structural cross-sections A–B and C–D presented in
Fig. 4 and of petrological samples p99 (Rahmati Ilkhchi et al. in press) and p378 (this study) used for PT
calculations.
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The micaschists intercalate with marbles and slices of basement orthogneiss, reflecting
complex structural relationships. The micaschists are similar in lithology to the
greenschist facies pre-Middle Triassic metasediments (see below), but show PT
conditions close to the basement rocks.
The pre-Middle Triassic metasedimentary rocks are subdivided into two
lithological sequences. The first, exposed along the southern border of the Shotur Kuh
basement, is represented by black meta-shales and schists with intercalation of marble
(Fig. 2).
.Fig. 2. Simplified stratigraphic column from the Shotur Kuh complex. Major tectonic events are also
indicated in the figure.
The second consists of metadolomite and calcitic marble with intercalation of calc-
schist at the bottom of the sequence and occurs in the northern part of the Shotur Kuh
(Fig. 1c). As no index fossils were found in these sequences, their precise age cannot
be determined. However, based on the presence of some corals, crinoid and Textularia
(Fig. 3a), the age of these subunits is probably older than middle Triassic (Rahmati-
Ilkhchi, 2002). Similar but unmetamorphosed sedimentary sequence in the south-west
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of the studied area (Fig. 1c), consists of dolomitic limestone, shale, sandstone and
limestone with Parafusulina sp., Schwagerina sp., Fusulinides and Climacamina sp
(Fig.3b), clearly indicating Permian age.
Fig. 3. Fossils from different cover formations of the Rezveh area. (a) Crinoid and Coral bearing
marble (Permian–Lower Triassic?), (b) biomicrite with fusulinds (Upper Permian), (c) recrystalized
biomicrite with Frondicularia sp., Miliolids, Cristellarias (Upper Triassic), (d) low-metamorphosed
clastic sedimentary rock with Crinoids (Jurassic), (e) sandy bio-pel-microsparite with Orbitolina sp.
(Lower Cretaceous), (f) biomicrosparite with Operculina (Paleocene?).
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Lithological correlation of the above-described formations suggests the presence of
one litho-stratigraphic unit of Permian–Middle Triassic age.
The Permian–Triassic metasediments are overlain by the Shemshak Formation
(e.g., Assereto, 1966; Stampfli 1978) with a hidden unconformity. This low-grade
metamorphosed formation, represented mostly by alternating meta-shales and meta-
greywacks, is interbedded with marble and conglomerates with pebbles of slates. The
limestones of the Shemshak Formation are fossiliferous and contain
Pseudocyclammina sp., Frondicularia sp., Cristellaria sp. and miliolids, (Fig. 3c)
suggesting Upper Triassic–Lower Jurassic age.
The Shemshak Formation is unconformably overlain by conglomerate with slaty
to very low-grade metamorphosed matrix that contains pebbles of the basement
granitoid and orthogneiss, and Permian–Middle Triassic and Shemshak Formation
marbles and meta-shales (Rahmati-Ilkhchi, 2002). The conglomerates are succeeded
by flysch sediments represented by alternating shales and sandstones interbedded with
micro-conglomerate and crinoid-bearing sandy limestone (Fig. 3d). This sedimentary
succession is very low-grade metamorphosed and based on fossil record (Cayexia Sp.,
Lithocudim Sp., Eggerlla Sp., Cristellaria Sp., Pseudocyclammina sp.,
Paleogaudryina sp.) can be assigned Middle Jurassic in age.
The Jurassic sedimentary rocks are overlain by very low-grade metamorphosed
massive limestone with alternating red conglomerates and sandstones at its base,
located in the south-west of the studied area (Fig. 1c). The limestone is fossiliferous
and contains Orbitolina sp., Pseudochoffatella sp., Pseudocyclammina sp.,
Nautiloculina sp. and Dictyoconus sp. (Fig.3e) suggesting Lower Cretaceous (Aptian–
Albian) age.
The Cretaceous limestone is unconformably overlain by well consolidated
unmetamorphosed conglomerates and red sandstone with Operculina (Fig. 3f).
Because of superposition, the age of these rocks is assumed to be Paleocene(Rahmati-
Ilkhchi, 2002). The Paleocene conglomerates are followed by poorly consolidated
unmetamorphosed Miocene conglomerates and sandstones. Their Miocene age is
based on lithological correlation of the conglomerate pebbles with Eocene volcanic
rocks in the surrounding area (Rahmati-Ilkhchi, 2002).
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3.3. Structural record
The Shotur Kuh complex has been affected by four major deformation events D1–D4.
The first two events are associated with the development of metamorphic foliation
while the later two events are characterized by small to large-scale folding. The
differences and similarities in structural record in the basement and overlaying
sedimentary rocks are described in the following sections.
Fig. 4. (a) Metamorphic foliation S1 affected by F2 folding and showing lack of retrogression within
amphibolite. Higher grade foliation S1 overprinted by lower grade foliation S2 within orthogneiss (b),
micaschist (c), Permian–Triassic cover (d, e) and Lower Jurassic Shemshak Formation (f).
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3.3.1. Orthogneisses and amphibolites
The basement rocks are affected by all four deformation events. The first deformation
event D1 is associated with the development of amphibolite facies metamorphic
foliation S1 defined by shape preferred orientation of muscovite, biotite and
recrystallized quartz aggregates in the orthogneiss and of amphibole and biotite or
plagioclase/amphibole banding in the amphibolite (Fig. 4a, b).
Fig.5. Deformation structures in the basement: (a) relics of S1 fabric within isoclinal folds F2 and
transposition of S1 into S2, (b) plagioclase sigma clasts within S2 indicating top-to-the-NW shear
sense, (c) isoclinally (F2) folded S1 in amphibolite, (d) isoclinal fold F2 of amphibolite “dyke” body,
(e) isoclinal fold F2 affected by F3 folding, (f) F4 kink band affecting the S2.
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The S1 foliation has been isoclinally folded, overprinted and/or completely transposed
into a new penetrative mylonitic foliation S2 (Fig. 5a–c). The S2 foliation is defined by
shape preferred orientation of quartz aggregates, fine-grained muscovite, rarely also
biotite and chlorite in orthogneiss (Fig. 4b) and shape preferred orientation of chlorite,
biotite and epidote in amphibolite. The S2 fabric is axial planar to the isoclinally folded
locally intercalating amphibolite ―dyke‖ bodies (Fig. 5d). D2 structures indicate
development and retrogression from amphibolite to greenschist facies conditions
manifested by the local occurrence of F2 isoclinally folded amphibolites lacking
retrogression (Fig. 5c and 4a) and subsequent formation of retrogressed axial planar S2
cleavages. The S2 fabric shows various orientations reflecting the domal structure of the
Shotur Kuh complex (Fig. 6), and it bears subhorizontal NW–SE trending stretching
lineation L2 defined by linear arrangement of micas and elongation of quartz
aggregates. The L2 is subparallel to axes of isoclinal F2 folds. Macroscopic kinematic
indicators within S2 fabric, such as sigma clasts (Fig. 5b), S-C and S-C` shear bands,
show top-to-the-Northwest shear sense. Subsequent deformation D3 is responsible for
folding of previous fabrics and results in the development of small to large-scale folds
(Fig. 5e) with subvertical NW–SE trending axial planes and subhorizontal axes (Fig. 6).
The latest ductile deformation D4 in the Shotur Kuh basement results in the
development of crenulation cleavage, kink bands (Fig. 5f) and open folds with
subvertical NE–SW trending axial planes and subhorizontal axes (Fig. 6).
3.3.1.1. Quartz deformation microstructures
Quartz deformation microstructures in the basement orthogneisses were investigated
in order to characterize temperature conditions of the observed macroscopic fabrics as
well as to evaluate regional microstructural differences. The orthogneisses exhibit two
distinct types of quartz microstructure related to recrystallization at different
temperature conditions. The first microstructural type, associated with S1 deformation
fabric, is characterized by relatively coarse-grained (ca 1 mm) recrystallized
quartz and higher temperature features such as lobate grain boundaries and island
grains (Fig. 7a). These features are typical for high temperature grain boundary
migration recrystallization mechanism (Poirier and Guillope, 1979, Urai et al., 1986),
i.e. the GBM recrystallization regime in quartz (Stipp et al., 2002).
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Fig.6. Structural map showing trend and dip of foliation S2 and lineation L2, and two structural
profiles; the WSW–ENE trending profile A–B and the NW–SE trending profile C–D. The data in the
pole figures cover the whole region and show orientation of following structures: sedimentary bedding
S0, D1 and D2 metamorphic foliations S1 and S2 and lineation L2, and D3 and D4 fold axial planes
AP3 and AP4 with fold axes FA3 and FA4. The pole figures are in lower hemisphere equal area
Schmidt projection. Contours are multiples of angular standard deviation above the uniform
distribution.
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According to the temperature-microstructure calibration of Stipp et al., (2002), this
microstructure is common for amphibolite facies metamorphic conditions. The second
microstructural type, associated with S2 fabric, is characterized by relatively fine-
grained recrystallized quartz and the development of core and mantle microstructure
(Fig. 7b, c). Such microstructure is typical for the subgrain rotation recrystalization
mechanism (White, 1979), i.e. the SGR recrystalization regime in quartz (Stipp et al.,
2002). According to the temperature-microstructure calibration of Stipp et al., (2002),
this microstructure is common for greenschist facies conditions.
Fig. 7. Quartz deformation microstructure in the basement orthogneiss showing two distinct
microstructures: (a) high temperature GBM (D1-related) and (b) lower temperature SGR (D2-related).
An example of overprinting relationship of the first D1 microstructure by the second D2 microstructure
is shown in (c). The grid in (d) shows spatial variations in an intensity of microstructure overprint
across the Shotur Kuh basement; the low and high intensity is exemplified in (a) and (b), respectively.
The micrograph in figure 7c shows clear overprint of the first higher temperature
D1 microstructure by the second lower temperature D2 microstructure. The degree of
microstructural overprint range from very low (Fig. 7a) to complete obliteration of the
first microstructure (Fig. 7b). We attempted to quantify the degree of microstructural
overprint on the basis of optical microscopy and determination of extreme cases of
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overprinting relationships (Low–High in Fig. 7a-c). The optical analysis of thin
sections collected across the Shotur Kuh basement (Fig. 7d) shows that the intensity
of microstructural D2 over D1 overprint increases from the central parts of the
basement towards the contact with the cover formations.
3.3.2. Micashists
Similarly to the basement rocks, the micaschists exhibit two metamorphic fabrics. The
higher grade S1 metamorphic foliation is present within relics of isoclinal F2 folds
while the rock is dominated by the lower grade S2 cleavage. The S1 fabric is defined
by shape preferred orientation of larger (ca 1 mm) muscovite and biotite grains which
are kinked and cleaved during the formation of S2 (Fig. 4c and d). Micaschists in the
vicinity of basement orthogneiss contain idioblastic garnets (up to 0.5 mm in size)
which together with the relicts of micaceous S1 foliation are enclosed within quartz
rich microlithons separated by S2 cleavage (Fig. 4c). The S2 cleavage dips to the
south or southwest at moderate angles (Fig. 6).
3.3.3. The para-autochthonous cover sequences
The differences in structural record within different litho-stratigraphic sequences of
the Shotur Kuh cover have been used to define three distinct tectono-stratigraphic
units; Permian–Lower Jurassic, Middle Jurassic–Cretaceous and Paleocene–Miocene.
The Permian–Lower Jurassic rocks record D1–D4 deformation events recognized
in the basement. The intensity of deformation increases towards the cover-basement
boundary and range from the undeformed meta-shales and marbles to intensely
deformed schists and marbles at the contact with the Shotur Kuh basement.
Deformation of the Permian–Triassic rocks at the contact with the basement is
characterized by boudinage of metadolomites enclosed by thin-bedded limestones and
schists. Similarly to the basement orthogneiss and micaschist, the Permian–Lower
Jurassic rocks are characterized by the presence of two metamorphic fabrics (Fig. 4e-
f). The greenschist metamorphic foliation S1 is sub-parallel to original sedimentary
stratification S0 as well as to lower grade metamorphic fabric S2 (cf. Fig. 8a, b). The
S1 foliation is defined by shape preferred orientation of larger (ca 0.5 mm) muscovite
and chlorite, and contains greenish biotite near the contact with the basement.
85
Fig. 8. (a) S1 and S2 fabrics affected by F3 folding in Upper Permian-Lower Triassic? schist, (b) S0-S1
fabrics within isoclinal folds F2 in Upper Permian-Lower Triassic? marble. (c) Middle Jurassic
conglomerate with basement orthogneiss pebbles. (d) Large-scale F3 fold affecting S0 in Middle
Jurassic flysch sediments: the inset shows S3 cleavage locally developed within folded shale strata. (e)
Angular unconformity between folded (D3) Cretaceous limestone strata and Paleocene conglomerate.
(f) F4 fold affecting S0 fabric in Paleocene conglomerate; orientation of the two limbs as well as of all
measured bedding planes is shown in pole figures.
86
The S2 fabric, defined by fine-grained white mica, is axial planar to locally observed
isoclinal folds F2 affecting the S0-S1 bedding-fabric (Fig. 8b). The F2 fold axes are
subparallel to the L2 lineation defined by linear arrangement of micas. The S0-S2
fabric within the Permian–Lower Jurassic rocks exhibits various orientations
depending on the location of the cover with respect to the basement so it reflects the
D2 related domal structure of the Shotur Kuh complex (Fig. 6).
The S0-S2 fabric orientations are modified by two subsequent folding phases D3 and
D4, which result in the development of small to large scale folds and crenulation
cleavages with steep NW–SE (D3) and NE–SW (D4) trending axial planes and
subhorizontal axes (Fig. 6).
The Lower Jurassic Shemshak Formation is directly overlain by very low-grade
metamorphosed Middle Jurassic conglomerates containing pebbles of basement
orthogneiss (Fig. 8c). The deformation fabric within orthogneiss pebbles is defined by
fine-grained muscovite and show similar microfabric features as the S2 foliation in
the basement orthogneiss. Bedding S0 in the Middle Jurassic–Cretaceous rocks is
only affected by the D3 and D4 folding events. D3 event is responsible for the
development of large scale folds (Fig. 8d) with subvertical NW–SE trending axial
planes and subhorizontal axes and locally leads to the development of spaced axial
cleavages S3 (inset in Fig. 8d). D4 event is characterized by development of small
scale folds and crenulation cleavages with subvertical NE–SW trending axial planes
and subhorizontal axes.
Palaeocene conglomerates overlie Cretaceous limestones with clear angular
unconformity (Fig. 8e) related to D3 folding of cretaceous limestone. The bedding in
the Paleocene–Miocene conglomerates is only affected by D4 folding, which results
in the development of tens of metre-scale gentle to open folds (Fig. 8f) with steep
NE–SW trending axial planes and subhorizontal axes.
3.4. Petrography and mineral composition
Chemical composition of most minerals was analysed at the Institute for Mineralogy
and Petrology in Graz using a JEOL 6310 scanning electron microscope (SEM)
equipped with wavelength- and energy-dispersive spectrometers. Standards were
pyrope (Mg, Al), adularia (K), rutile (Ti), tephroite (Mn), jadeite (Na, Si), and
andradite (Fe, Ca). Na was measured by wavelength-dispersive spectrometers, and
87
operating conditions were 15 kV and 10 or 15 nA, with 20 s counting time.
Representative mineral analyses are given in Table 1. Mineral abbreviations used in
the text and figures are after Kretz (1983).
3.4.1. Orthogneisses and amphibolites
Detailed petrology of basement rocks, mainly of orthogneisses and amphibolites, is
given in Rahmati-Ilkhchi et al. (in press). The orthogneisses are medium- to coarse-
grained and typically consist of feldspars, quartz and biotite (XMg = 0.26–0.36) with
small amounts of muscovite, garnet, allanite-epidote and accessories of apatite, zircon
and monazite. The garnets are rich in almandine (alm59-73, grs6-18, py14-20, sps1-9) with
mostly flat zoning profiles, although in some samples (e.g. p99 for location see Fig. 1)
exhibit prograde compositional zoning (for details see Rahmati-Ilkhchi et al. in press).
Amphibolite consists of plagioclase (an18-30), pargasite-tschermakite and locally also
garnet (alm49-64, grs31-34, py3-9 and sps1-9). In one case gedrite with kyanite, phengite
and rutile were also found. The basement rocks are retrogressed to various degrees.
Feldspars are partly replaced by fine-grained white mica. Biotite is replaced by
chlorite with rutile and epidote forms within or adjacent to biotite. Garnet is partially
replaced by fine-grained brown to green biotite and chlorite mixture along cracks and
garnet rims. Amphibole is partly replaced by chlorite, biotite and epidote.
3.4.2. Micaschist
The micaschists are strongly foliated and contain two metamorphic fabrics (Fig. 4c,
d). These rocks consist of quartz, white mica, biotite, sometimes garnet and variable
amounts of plagioclase and they mostly contain graphitic substance with fine-grained
rutile and other Ti phases. Retrogression in the micaschists is characterized by
replacement of garnet into calcite and chlorite, biotite by chlorite, and plagioclase by
fine-grained white mica.
Garnet from micaschist (alm49-70, grs16-18, py3-10, sps1-31) shows prograde zoning
with rimward increase of XMg and decrease of spessartine content. Some samples
contain two compositional varieties of garnet (Fig. 9a) represented by garnet (I) in the
cores (alm58-78, grs8-13, py5-11, sps5-24) and garnet (II) in the rims (alm75-78, grs7-12, py11-
13, sps0-5).
88
Fig. 9. (a) BSE image of micaschist showing two
compositional varieties of garnet core (I) and rim (II).
The inset (b) shows detail of zoned garnet with
compositional profiles in (c). Sharp change in Grs and
Alm contents and a weak decrease of Py and XMg values
can be seen at contact between garnet (I) and (II). Sps
content generally decrease from core to rim, but small
disturbance is visible at the contact of both garnets.
Fig. 10. Photomicrograps of muscovite porphyroblasts with inclusion trails of quartz (a, c) and
graphite (b, d). Note concentration of quartz and/or graphite highlight the hourglass structure of
chiastolite (andalusite) or sector trilling in cordierite.
Although both garnets show prograde compositional zoning characterized by an
increase of XMg and decrease of sps, garnet (I) exhibits a decrease of calcium and
89
increase of iron towards the rim, while this zoning pattern is reversal in the garnet (II)
(Fig. 9b). Depending on the whole rock composition, plagioclase composition range
from pure albite to andesite (an40). Biotite has uniform composition with XMg = 0.52–
0.55 and the Si and paragonite contents in muscovite correspond to 3.0 a.p.u. and 11–
16 mol %, respectively.
One of the studied garnet-free samples contains large (up to 1 mm) rhomboidal
muscovite porphyroblasts within fine-grained matrix. The porphyroblasts show
synkinematic growth indicated by the presence of graphite and quartz inclusion trail
patterns with s-shaped porphyroblast geometry (Fig. 10). The distribution of quartz
and graphite inclusions within these porphyroblasts (Fig. 10a-d) remains an hourglass
structure of chiastolite or sector trilling in cordierite and suggests epitaxial
replacement of these minerals by muscovite.
3.4.3. The para-autochthonous cover rocks
The above-mentioned tectono-stratigraphic division of the cover into Permian–Lower
Jurassic, Middle Jurassic–Cretaceous and Paleocene–Miocene units is consistent with
the observed differences in metamorphic grade of the three units. The Permian–Lower
Jurassic meta-shales and schists, characterized by the presence of two metamorphic
fabrics (Fig. 4e, f), consist of quartz, two varieties of white mica, chlorite and
graphitic substance. Near the contact with the basement they also contain greenish
biotite. The degree of recrystallization and the grain size of quartz and micas within
both fabrics decreases away from the basement/cover boundary towards the younger
members of this tectono-stratigraphic unit. The Middle Jurassic–Cretaceous rocks
indicate very low-grade metamorphism characterized by the presence of ultra fine-
grained mica and chlorite along the bedding planes as well as spaced cleavage S3. The
uppermost Paleocene–Miocene conglomerates are characterized by unmetamorphosed
matrix.
3.5. PT conditions of metamorphism
PT conditions for the basement orthogneisses and amphibolite were obtained by using
various exchange thermobarometres and thermodynamic modelling (Rahmati-Ilkhchi
et al., in press). They show a prograde metamorphism that reached 8.5 kbar/650 °C in
orthogneisses, while the amphibolite yielded slightly higher pressures of 8–10 kbar.
90
In contrast to basement orthogneisses and amphibolite with only amphibolite
facies mineral assemblages, the micaschists bear evidence for polymetamorphic
evolution prior to amphibolite facies metamorphism. In addition to two
microstructural and compositional varieties of garnet, the muscovite porphyroblasts
showing the hourglass structure with graphite and quartz probably represent former
chiastolite that was replaced by a single crystal of muscovite during later
metamorphic process. Based on the stability field of minerals in graphite-bearing
pelitic rocks (Likhanov et al., 2001), the presumed andalusite without garnet and
cordierite suggests low-pressure (below 3 kbar) metamorphism at temperatures ~500–
550 °C. To constrain the garnet growth-related metamorphic evolution of micaschist,
we used microstructural relationships in combination with phase equillibrium
modelling and PT section approach (Connolly 2005), biotite-garnet exchange
thermometery (Ganguly et al., 1996; Holdaway, 2000) and garnet-biotite-plagioclase-
quartz barometry (Wu et al., 2006).
Fig. 11. PT sections (MnNCKFMASH system) constrained for micaschist sample p378 (for location see
Fig. 1). (a) shows PT section calculated with the XRF whole rock composition covering the growth of
core garnet (I) and (b) shows PT section calculated with the fractionated whole rock composition (see
text for details) covering the growth of rim garnet (II). Solid black circles indicate intersection of
garnet compositional isopleths (XMg, grs and sps) for the core and rim compositions of garnet (I) and
(II). Bulk rock composition in molar% is indicated in each PT section.
The PT sections were calculated in the thermodynamic software Perple_X
(Connolly 2005: version 07), with internally consistent thermodynamic dataset of
91
Holland & Powell (1998; 2003 upgrade). As for the whole-rock composition, mineral
chemistry (particularly high Mn content in garnets) and modal proportion of mineral
phases, the P–T sections were calculated in the MnO-Na2O-CaO-K2O-FeO-MgO-
Al2O3-SiO2-H2O (MnNCKFMASH) system with excess water. Because the
micaschist contains relatively high proportion (5–6 modal%) of garnet with two
distinct types of compositional zoning, two PT sections reflecting fractionation of
bulk rock composition during garnet (I) and garnet (II) growth have been calculated.
To construct the PT section for garnet (I) (Fig. 11a), we used the whole rock
composition obtained by X-ray fluorescence (XRF) analysis. The second PT section
for the garnet (II) (Fig. 11b) was constructed on the basis of effective bulk rock
composition. The effective bulk rock was obtained by extracting the garnet (I) from
the whole rock composition. The modal proportion of garnet (I) was determined by
image analysis from several BSE images of the studied thin section. In addition to the
plagioclase solution model by Newton et al. (1980) and the garnet model by Berman
(1990), we used the solution models of White et al. (2001) for NCKFMASH end-
member phases expanded by the Mn end-members (Tinkham et al. 2001).
The PT sections calculated for sample p378 (for location see Fig. 1) indicate that
garnet (I) and (II) compositions plot into Grt-Bt-Ms-Chl-Pl-Q stability field, which is
in a good agreement with observed mineral assemblage (Fig 11). Intersections of
isopleths for garnet (I) core and rim (Fig. 9c) suggests an increase in PT conditions
from 4.4 kbar/525 °C to 5.4 kbar/585 °C (Fig. 11a, Table 1). An increase in PT
conditions from 4.9 kbar/560 °C to 7.3 kbar/600 °C is also recorded by the core and
rim composition of garnet (II) (Fig. 11b, Table 1). These values indicate that the
garnet (II) core shows lower pressure and temperature than the rim of garnet (I) (Fig.
12). The thermometry of Ganguly (1996) and Wu et al. (2006) indicates slightly
higher temperature of 63425 °C and 65112, respectively, for the rim composition
of garnet (II) and matrix biotite (Table 1). The garnet-biotite-plagioclase-quartz
barometry of Wu (2006) gave pressure of 8.10.9 kbar (Table 1). All of these PT
estimates are similar to those obtained for the basement orthogneisses and amphibolite
(Rahmati-Ilkhchi et al., in press).
PT conditions of the Permian–Lower Jurassic have not been treated by mineral
composition or other relevant method; however, the newly growing minerals (white
92
mica, chlorite, calcite and locally albite) suggest that the metamorphism of these rocks
correspond to greenschist to lower part of the greenschist facies conditions. Formation
of chlorite porphyroblasts with white or brown mica (mixed-layer phyllosilicate),
occurring in the Shemshak formation, suggest very low-grade metamorphic
conditions below the greenschist facies (Fraceshelli et al., 1986, Arkai et al., 2003).
Table 1. Microprobe analyses of minerals in garnet-bearing micaschist (sample p378)
--------------------------------------------------------------------------------------------------------
-------
Grt(I)-c Gr (I)-r Grt(II)-c Gr(II)-r Bt Ms Pl
SiO2 37.12 37.45 38.03 37.77 36.27 47.31 59.63
TiO2 0.05 0.00 0.06 0.00 1.65 0.92 0.01
Al2O3 20.34 20.62 20.88 21.24 17.14 33.92 25.15
FeO 28.51 30.07 34.74 34.01 20.43 1.36 0.19
MnO 7.49 5.84 1.36 0.36 0.12 0.00 0.10
MgO 2.04 1.91 2.85 3.34 9.05 1.17 0.02
CaO 3.68 3.80 2.55 3.84 0.07 0.03 7.26
Na2O 0.08 0.04 0.00 0.00 0.11 0.93 7.59
K2O 0.04 0.00 0.03 0.00 9.23 10.11 0.04
99.35 99.74 100.51 100.55 94.05 95.75 99.98
Si 3.019 3.028 3.037 3.004 2.889 3.137 2.660
Ti 0.003 0.000 0.004 0.000 0.099 0.046 0.000
Al 1.952 1.979 1.988 1.991 1.609 2.650 1.322
Cr 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Fe3+ 0.094 0.063 0.042 0.044 0.000 0.000 0.007
Fe2+ 1.848 1.985 2.305 2.218 1.361 0.075 0.000
Mn 0.516 0.403 0.093 0.024 0.008 0.000 0.004
Mg 0.248 0.232 0.344 0.396 1.075 0.115 0.001
Ca 0.321 0.331 0.221 0.327 0.006 0.002 0.347
Na 0.013 0.007 0.000 0.000 0.016 0.120 0.657
K 0.004 0.000 0.003 0.000 0.938 0.855 0.002
Xmg 0.118 0.105 0.130 0.151 0.441
Alm 63 67 78 75
Pyr 8 8 12 13
Grs 11 11 7 11
Sps 18 14 3 1
PT Conditions, calculated garnet-biotite thermometry and garnet-biotite-plagioclase-
quartz barometry
Ganguly
Wu(GB-
GBPQ)
Wu(GB-
GBPQ)
T 63425 65112 8.10.5
93
3.6. Discussion
3.6.1. Polymetamorphic record in the Shotur Kuh complex
As mentioned above, the basement orthogneiss and amphibolites show relatively
simple metamorphic history characterized by prograde metamorphism reaching
amphibolite facies conditions. The core to rim compositional change in garnets from
the orthogneiss documents PT increase from 7 kbar/600 °C to 8.5 kbar/650 °C while
amphibolite indicate slightly higher pressures of 8–10 kbar (Rahmati-Ilkhchi et al., in
press). Compositional zoning of garnets in orthogneiss (sample p99) and micaschist
(sample p378, for location see Fig. 1) indicate similar dP/dT gradients and PT
conditions reaching kyanite stability field (Fig. 12). Indeed the presence of kyanite
has been documented in amphibolites (Rahmati-Ilkhchi et al., in press) and it is
consistent with Barrovian metamorphic field gradient of 20–22 °C km-1
obtained from
the PT sections (Fig. 12).
Fig. 12. Summary of PT conditions calculated by using phase equilibrium modelling for basement
orthogneisses sample p99 (see Fig. 1 for location) (solid curve, Rhamati-Ilkhchi et al., in press) and for
micaschist sample p378 (dashed and dotted curves, this study). PT paths (I and II) in micaschists are
based on PT sections (Fig. 10) constructed for the core garnet (I) and rim garnet (II). Star indicates
possible thermal event based on the pseudomorphs of muscovite after andalusite or cordierite.
94
In contrast to the orthogneisses and amphibolite, the micaschists show evidence
for polymetamorphic evolution. The earliest metamorphic phase in these rocks is
manifested by large rhomboid porphyroblasts of muscovite with hourglass structure or
sector trilling inclusion pattern, which probably represents pseudomorphs after
chiastolite or cordierite (Fig. 9). As these high-temperature and low-pressure
phenomena were observed only in one case, they might be related to a local contact
metamorphic event (Fig. 12). Subsequent metamorphism in these rocks is mainly
represented by a growth of two compositional varieties of garnet (Fig. 10). By using
garnet compositional isopleths in the calculated PT sections (Fig. 11), the
compositional zoning of core garnet (I) indicates PT increase from 4.4 kbar/525 °C to
5.4 kbar/585 °C and the rim garnet (II) indicates PT increase from 4.9 kbar/560 °C to
7.3 kbar/600 °C (Fig. 12). As summarized in Fig. 12, both garnets indicate prograde
metamorphism although the prograde PT path segments for each garnet differ in
dP/dT gradient exhibiting lower gradient for garnet (I) and higher gradient for garnet
(II). Furthermore, the reversal in compositional zoning trends of calcium and iron
separating the garnet (I) from garnet (II) is marked by a disturbance of manganese and
magnesium, which in the calculated PT sections suggests slight (0.5 kbar and 20 °C)
drop in pressure and temperature between the garnet (I) rim and the garnet (II) core. It
is not clear, whether this compositional change occurs during single metamorphic
event as a result of progressive burial, or it relates to two distinct metamorphic events
separated by a period of cooling/exhumation. The observed element disturbance,
including Mn contents, at the garnet (I)/(II) interface suggests partial resorbtion of the
garnet (I) and subsequent formation of garnet (II) supporting a separate metamorphic
events. However, such disturbance in compositional zoning could also occur during a
single metamorphic event, if the rock had crossed the staurolite stability field. More
detailed study on the micashist is needed to confirm one or the other alternative.
Another question that remains unclear is the lithological similarity of the micaschist
(intercalating marble) with the Permian-Triassic cover rocks. The cover rocks,
however, show only greenschist facies conditions where greenish biotite is present at
the contact to the micaschist and basement rocks and the degree of metamorphism
decrease upwards. There are two alternatives to explain the metamorphic gap between
micaschists and the Permian-Triassic metasediments: 1) the micaschists represent an
older (pre-Permian) sedimentary formation that was eventually affected by a contact
metamorphism prior to its amphibolite facies metamorphism, or 2) they derived from
95
the Permian-Triassic strata and their present juxtaposition is a result of extensional
detachment shearing at the basement/cover contact (see below). The second
alternative is supported by the similarities in lithology and deformation character of
the micaschist and overlaying Permian-Triassic rocks.
3.6.2. Tectonic evolution of the Shotur Kuh complex
D1 deformation is associated with the development of high grade metamorphic
foliation S1 in the basement and Permian–Lower Jurassic cover. In the orthogneiss
and micaschist, the S1 fabric contains prograde garnets indicating that it formed
during progressive burial of the Shotur Kuh complex. As the S1 fabric exhibits
decrease in metamorphic grade from amphibolite facies in the basement to lower
greenschist facies towards the Lower Jurassic Shemshak Formation in the cover, the
D1 event is probably associated with a period of crustal thickening (Fig. 13). This is
supported by the Barrovian type metamorphic field gradient derived for orthogneiss
and micaschist imposing collision of continental crustal blocks. The original
orientation of S1 fabric is difficult to predict due to an intense overprint by subsequent
deformation events. However since S1 is suggested to have formed during a crustal
thickening stage, it is likely that it was initially steep. This is supported by the
presence of isoclinal F2 folds, which document angular relationship between S1 and
subhorizontal, axial planar fabric S2.
D2 deformation is associated with vertical shortening of S1 fabric and formation
of axial planar cleavages S2 whose orientation reflects domal structure of the Shotur
Kuh complex (Fig. 6). Microstructural analysis of D2 deformation structures (Fig. 4)
indicate that they developed at various PT conditions ranging from amphibolite to
greenschist facies conditions in the basement and greenschist to very low-grade
conditions in the cover. This is manifested by the lack of retrogression within
isoclinally folded amphibolite (Fig. 4a) and formation of the greenschist facies S2
cleavage in the orhogneiss (Fig. 7b and 4b), micaschist and Permian–Lower Jurassic
metasediments (Fig. 4c-f). In the basement, however, the spatial proximity of D2
structures with contrasting metamorphic grade suggests progressive decrease in PT
conditions related to exhumation of the Shotur Kuh complex. In this context, the D2
deformation records buyoancy driven lower crustal upflow, which follows a period of
thermal relaxation of previously thickened Shotur Kuh crust. The excess in
96
gravitational potential energy led to a local extensional collapse and updoming of the
Shotur Kuh complex (Fig. 13). At the same time, we cannot rule out the hypothesis
that this process is part of a large-scale extension in the Great Kavir Block.
Regional variations in the degree of D2 deformation overprint across the Shotur
Kuh basement manifested by quartz deformation microstructures (Fig. 7d) indicate
that the intensity of D2 overprint increases towards the basement/cover contact. This
observation is in an excellent agreement with rather sharp metamorphic gradient at the
basement/cover contact thus proposing the existence of low angle detachment shear
zone juxtaposing the higher-grade basement core and lower-grade cover. The
updoming/unroofing of the basement core is associated with the development of NW–
SE trending lineation and top-to-the-NW kinematic indicators documenting the north-
westward transport of the upper crustal portions (Fig. 13). In this concept the
alternation of basement and cover rocks in the east of the Shotur Kuh complex (see
Fig. 1) represent extensional duplexes
D1 and D2 structural-metamorphic record has been equally recognized in the
basement and the Permian–Lower Jurassic cover while the Middle Jurassic
conglomerates, directly overlying Lower Jurassic meta-shales of the Shemshak
Formation, are virtually unmetamorphosed and contain pebbles of basement
orthogneiss. This indicates that D1 burial and D2 exhumation of the Shotur Kuh
complex must have occurred close to the Lower/Middle Jurassic boundary and in
relatively quick succession. Such time constraint roughly corresponds to the Ar-Ar
total gas cooling age of 160.4 ± 2.8 Ma obtained from muscovite in the basement
orthogneiss (Rahmati-Ilkhchi et al., in press).
D3 deformation, equally affecting basement and Permian–Lower Cretaceous
cover, is associated with folding induced by subhorizontal shortening in the NE–SW
direction (Fig. 13). The angular unconformity between upright stratification of Lower
Cretaceous limestone and horizontal stratification of Paleocene conglomerate (Fig.
8e) suggests Upper Cretaceous age of the D3 event.
D4 folding affects all the rock units of the Shotur Kuh complex, including
Paleocene and Miocene, thus indicating post-Miocene age of this event. Steep NE–
SW trending axial planes of the F4 folds indicate subhorizontal shortening in the
NW–SE direction (Fig. 13).
97
Fig. 13 Sketch showing summary of tectonic evolution of the Shotur Kuh complex
98
3.6.3. Regional implications
The Cimmerian tectono-metamorphic processes that occurred in the Central Iranian
domain and Sanandaj-Sirjan zone are mostly related to the closure 1) of Paleo-
Thethys in Carboniferous – Mid- to Late Triassic (Early-Cimmerian) event (Paul et
al., 2003; Ramezani and Tucker, 2003; Sheikholeslami et al., 2008; Bagheri and
Stampfli, 2008), and 2) of Neo-Tethys in Cretaceous–Eocene (Late-Cimmerian–Early
Alpine) event (Ramezani and Tucker, 2003; Bagheri and Stampfli, 2008). However,
new geochronological data of Jurassic age in this region (Khalatbari-Jafari, 2003;
Rachidnejad-Omran et al., 2002; Sheikholeslami et al., 2003, 2008; Falznia et al.,
2007; Bagheri and Stampfli, 2008; Rahmati-Ilkhchi et al. in press) led to
consideration of an older concept of Mid-Cimmerian event of Aghanabati (1992) and
several other evolutionary scenarios. For example, the Ar-Ar ages spanning between
187–156 Ma from the Anarak-Jandaq terrane of the Yazd block have been interpreted
as a cooling subsequent to the Late Triassic (Early-Cimmerian) or even older
metamorphic phase (Bagheri and Stampfli, 2008). In contrast, the zircon SHRIMP U-
Pb (187±2.6 Ma) and monazite CHIME U-Th-Pb (180±21 Ma) ages from partial
melts within the Qori complex in the Sanadaj-Sirjan zone have been interpreted as an
extended phase of the Early-Cimmerian metamorphism (Falznia et al., 2007; 2009).
According to Falznia et al. (2009), this metamorphism relates to collision between the
Central Iranian domain and Arabian plates prior to opening of the Neo-Tethys Ocean,
marked by intracontinental rifting and related an orogenic magmatism in Middle
Jurassic (173±1.6 Ma). In this concept, the Upper Jurassic (147±0.76 Ma) arc-related
metamorphism documents the beginning of a young and buoyant Neo-Tethys
subduction. On the other hand, recent paleogeographic reconstructions assume either
an independent Jurassic–Cretaceous drift of Sanandaj-Sirjan zone within the Neo-
Tethys realm (Golonka, 2004) or a drift of the whole Cimmerian block within the
frame of closing Paleo-Tethys in the north and simultaneous opening of Neo-Tethys
in the south (Stampfli and Borel, 2004; Bagheri and Stampfli, 2008). Both these
scenarios impose relatively long-term (Triassic–Cretaceous) existence of the Neo-
Tethys Ocean that during its northward subduction facilitated formation of several
back-arc basins (Bagheri and Stampfli, 2008).
Despite temporal proximity of Lower/Middle Jurassic metamorphism in the Qori
and in the Shotur Kuh complexes, the burial and exhumation of the Shotur Kuh
99
complex cannot be related to an extended period of Early-Cimmerian event. This is
because the D1 and D2 deformation and metamorphism in the Shotur Kuh affected
both basement and Permian–Lower Jurassic sediments including the Shemshak
Formation, which represents mollasic/forland deposition post-dating the Early-
Cimmerian event (Assereto, 1966; Stocklin, 1968; Alavi 1996). Following Alavi
(1992), who established the Middle Jurassic orogenic event in the Alborz region on
the basis of tectono-metamorphic overprint of the Shemshak Formation, we attribute
the metamorphism in the Shotur Kuh complex to the Mid-Cimmerian orogeny. The
question remains, whether the Mid-Cimmerian event represents a broader orogenic
process or it related to a series of Middle Jurassic–Cretaceous back-arc extensional
events and compressional cycles controlled by northward subduction of the Neo-
Tethys Ocean (Bagheri and Stampfli, 2008). Indeed, some mafic and ultramafic rocks
near Kuh Siah Poshteh in Moalleman and small remnant of ophiolite near the
Khondar anticline in Meyamey occur about 100 km to the SW and 95 km to the NE
from the Shotur Kuh complex, respectively. The age data from these complexes are
not available, but they are considered to represent remnants of ophiolites of pre-
Cretaceous time (Jafariyan, 2000, Amini Chehragh, 1999). On the other hand, the
Lower/Middle Jurassic extension/exhumation in the Shotur Kuh complex overlaps
with the intracontinental rifting within the Cimmerian block (e.g. Falznia et al., 2009)
and thus suggests an independent Mid-Cimmerian orogenic event prior to the major
extensional process in the back-arc domain.
The Cretaceous to lower Eocene Sabzevar and Nain ophiolite complexes (Rossetti
et al., 2009; Davoudzadeh, 1972) occurring to the NE and SW from the Shotur Kuh,
respectively, are roughly NW–SE trending. Their alignment reflects the Late-
Cimmerian – Early Alpine, probably NE–SW directed (cf. Sheikholeslami et al.,
2008) closing of several back-arc basins across the Cimmerian domain at the late
stages of Neo-Tethys closure. These directions are in perfect agreement with the
observed Late Cretaceous NE–SW shortening D3 in the Shotur Kuh complex and
imply a propagation of the deformation front inland.
The Late Miocene NW–SE shortening D4 in the Shotur Kuh complex is
associated with the Alborz and Zagros phase of the Late Alpine collision between the
Arabian and Eurasian plates (e.g. Alavi, 2004; Guest et al., 2006). Although our D4
compressional stress deviates from the roughly N–S directed convergence of the two
plates, local stress field deviations are expected in case of a complex geometry of
100
interacting crustal segments with different rheological properties (Soukotis et al.,
2000).
3.7. Conclusions
On the basis of structural, metamorphic and stratigraphic record, we distinguish
following stages of tectonometamorphic evolution of the Shotur Kuh complex:
1) Lower/Middle Jurassic collisional crustal thickening led to Barrovian
metamorphism with field metamorphic gradient of 20–22 °C km-1
that affected
Neoproterozoic–Cambrian basement granitoids as well as Permian–Lower Jurassic
cover rocks including the Shemshak Formation.
2) Subsequent exhumation of the Shotur Kuh complex marked by the deposition of
Middle Jurassic conglomerates with pebbles of basement orthogneiss and Permian–
Lower Jurassic cover, is associated with an upflow of lower crust resulting in
updoming of the basement core and its top-to-the-Northwest unroofing along a low
angle detachment shear zone at the basement/cover boundary. Both processes are
affiliated to the Mid-Cimmerian event, which in turn might be associated with a series
of Jurassic–Cretaceous collisional and extensional processes in the back-arc domain
controlled by the Neo-Tethyan subduction.
3) Upper Cretaceous folding event is probably related to the Late-Cimmerian–Early
Alpine orogeny which resulted from the convergence of Arabian and Eurasian plates,
and the Cenozoic closure of the Neo-Tethys oceanic tract(s) by subduction.
4) Late Miocene to post-Miocene folding event is probably associated with Late
Miocene Zagros and Alborz phase of N–S convergence between Arabian and
Eurasian plates. This shortening event could be also combined with a left-lateral
activity along the Great Kavir fault bounding system.
101
3.8. References
Aghanabati, A., 1992. Representing the Mid-Cimmerian tectonic event in Iran.
Scientific Quarterly Journal, Geosciences, Vol. 6. Geological Survey of Iran.
Alavi M., l992 Thrust tectonics of the Binalood region. NE Iran: Tectonics I.I 360-
370.
Alavi M., l996 Tectonostratigraphic Synthesis and Structural style of the Alborz
Mountain System in northern Iran. J. Geodynamics Vol. 21, No. 1, pp. l-33.
Alavi, M., 2004. Regional stratigraphy of the Zagros fold-thrust belt of Iran, and its
proforeland evolution. American Journal of Science 304, 1–20.
Árkai, P., Faryad, S.W., Vidal, O., Balogh, K. (2003): Very low-grade metamorphism
of sedimentary rocks of the Meliata unit, Western Carpathians, Slovakia:
implications of phyllosilicate characteristics - International Journal of Earth
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106
CHAPTER 4
4.1. Summary and further research directions
Geochemical and geochronological data from igneous and metaigneous rocks in
Iran show that the Neoproterozoic arc magmatism significantly contributed to the
formation and consolidation of the Central Iran microplate (Ramezani and Tucker,
2003). This is confirmed also by our research results, indicating that the arc-related
igneous rocks are present not only along the boundaries of the Central Iran block but
also in all parts of this microplate. Because most of the Iran territory is represented by
the Mesozoic sedimentary sequences, which form either thrust belts or covers of the
basement blocks, it is difficult to obtain data about metamorphism and deformation of
the Neoproterozoic crust and to reconstruct its pre-Mesozoic metamorphic history.
Recent advances in getting various geochronological data (Table 1, Fig. 7, Chapter 1)
from different geological units in Iran confirmed that the Central Iran basement units
not only were affected by the Upper Cretaceous metamorphism and deformation but
also experienced older (Lower, Middle, Late Cimmerian and Variscan) metamorphic
processes. Most crystalline complexes, simply referred to as the pre-Cambrian
basement units in Iran (Berberian et al., 1981), therefore need reinterpretation and
modification in order to clear their tectono-metamorphic history and their relations to
individual crustal blocks or metamorphic belts.
Metamorphic petrology from the Shotur Kuh complex showed a Middle-Jurassic
amphibolite facies metamorphism that affected both the Neoproterozoic igneous rocks
and adjacent sedimentary sequences. Based on their metamorphic mineral
assemblages, estimated PT conditions, and structural analyses, at least three
metamorphic and four deformation events can be recognized in the basement rocks.
Evidence for the first two metamorphic events (low-pressure/medium-temperature
and medium-pressure epidote amphibolite facies metamorphism) were recognized
only in the metapelitic rocks (micaschists) adjacent to the orthogneisses and
amphibolite. Based on the microfabrics (inclusions pattern of graphite and quartz) in
muscovite porphyroblasts (pseudomorph after andalusite?), the first event related to a
local thermal (contact) metamorphism. The second epidote-amphibolite facies event
was recognized by the presence of two garnet generations (I and II) that are separated
by a compositional gap. This metamorphism had prograde character with a
temperature/pressure range from 525 °C/ 4.4 kbar to 585 °C / 5.4 kbar. The second
107
garnet started to grow at 560 °C/ 4.9 kbar and reached 600 °C/ 7.3 kbar, which
corresponds well with PT conditions of the Middle-Jurassic amphibolite-facies
metamorphism established in the adjacent metaigneous rocks.
Regarding two (thermal and epidote-amphibolite facies) metamorphic events on
the one hand and lithological similarity between the micaschists and the Pre-Middle-
Triassic phyllites on the other hand, two alternative interpretations can be considered
for metamorphic evolution of the Shotur Kuh complex. The first interpretation
assumes a Pre-Cambrian low- to very-low-grade metamorphic basement into which
the igneous rocks were intruded and which resulted in only a thermal overprint in the
micaschists. The epidote-amphibolite facies metamorphism, separated by pressure and
temperature discontinuity from the Middle-Jurassic amphibolite-facies
metamorphism, could relate to the Variscan event that is known from the Anarak area
(Baghari and Stampfli, 2008). The whole complex, including Pre-Middle Triassic
cover rocks, was later (during the Middle Jurassic) metamorphosed in amphibolite
facies conditions. Preservation of prograde zoning in both garnet generations from the
micaschist suggests that both metamorphic events were relatively short in order to
totally equilibrate the mineral assemblages and to homogenize garnet zoning. As we
could not recognize textural evidence for the epidote amphibolite facies events in the
metaigneous rocks, the occurrence and age relations of this event in the Shotur Kuh
complex need further investigation.
The second interpretation is based on lithological parallelization of the
micaschists with the Permian-Triassic cover sequence. In this case, the two garnets
should have formed during a single (Middle-Jurassic) metamorphism. If this is true,
large-scale burial tectonic processes, which resulted in the formation and tectonic
juxtaposition of amphibolite and greenschist facies rocks in the same stratigraphic
unit, can be assumed. In addition to the difference in metamorphic grade (amphibolite
facies micaschists and greenschist facies phyllite), the weakness of this interpretation
is in the lack of arguments to explain the compositional gap between garnets I and II
that gives higher PT conditions for garnet I rim and lower for garnet II core. If the
chaistolic-like structures are after andalusite or cordierite, their formation also needs a
thermal event prior to amphibolite facies metamorphism.
The Barrovian-type Middle-Jurassic metamorphism in the metaigneous and
sedimentary rocks suggests crustal thickening, which is a typical feature in collision
for orogenic belt. This metamorphism was associated with D1 and D2 deformations,
108
which resulted in a high-grade metamorphic foliation S1 and vertical shortening of S1
fabric with axial planar cleavages S2, respectively, in the basement and overlaying
Permian-Lower Jurassic metasediments. Because the Paleotethys-related orogenic
processes ended in the Early to Middle Triassic (Bagheri and Stampfli, 2008),
formation of amphibolite facies metamorphism in the Shotur Kuh complex could
relate to closure of the Neotethys. This oceanic basin started to close by northward
subduction beneath the Iranian Microplate already in the Upper Triassic and resulted
in the formation of a series of back-arc basins. Recent geochronological data from the
Sabzevar area (Rossiti et al., 2009) show that the opening of the Sabzevar back-arc
basin should have occurred prior to the Upper Cretaceous (Bagheri and Stampfli,
2008), while some of these rocks suffered by Lower Cretaceous metamorphism. It is
not clear whether the Middle-Jurassic metamorphism in the Shotur Kuh complex
related to closure of the Sabzevar back-arc basin or possibly to another back-arc
basin, which could produce the Kuh Siah Poshteh mafic and ultramafic rocks (100 km
WSW from the Shotur Kuh complex). In both cases, the geochronological data from
the Shotur Kuh and other metamorphic units in Central suggest that the collisional
tectonics processes that were related to closures of the Neotethys and their back-arc
basin, started already in the Middle Jurassic.
In summary, the new data on timing of magmatism, metamorphism, and
deformation of the Shotur Kuh complex open several significant questions with regard
to the geodynamic evolution of the Great Kavir block in Central Iran. Parts of this
block were affected by metamorphism and deformation related to closure of the
Neotethyan ocean and its associated back-arc basins. One important question that
should be solved is the possible existence of Pre-Middle-Jurassic metamorphism in
the Shotur Kuh complex and its relation to other units in Central Iran. Although some
Jurassic ages of metamorphism are present in the Snandaj-Shirjan zone, the Middle-
Jurassic metamorphism in the Shotur-Kuh complex should be verified also in relation
to closure of the Sabzevar or possibly other back-arc basins.
109
4.2.References:
Bagheri, S., Stampfli, G.M., 2008. The Anarak, Jandaq and Posht-e-Badam
metamorphic complexes in central Iran: New geological data, relationships and
tectonic implications. Tectonophysics 451:123–155.
Berberian, M., King, G.C.P., 1981. Towards a Paleogeography and Tectonic
Evolution of Iran. Can J Earth Sci 8:210–265.
Ramezani J, Tucker RD, 2003. The Saghand Region, Central Iran: U–Pb
geochronology, petrogenesis and implications for Gondwana tectonics. Am J Sci
303: 622–665
Rossetti, F., Nasrabady, M. Vignaroli, G., Theye, T., Gerdes, A. Razavi, M.H. and
Vaziri. H.M. 2009. Early Cretaceous migmatitic mafic granulites from the
Sabzevar Range (NE Iran): implications for the closure of the Mesozoic peri-
Tethyan oceans in central Iran. Tectonics (in press).
Institute of Petrology and Structural Geology
Faculty of Science
Charles University
Prague, Czech Republic
METAMORPHISM AND GEOTECTONIC POSITION OF
THE SHOTUR KUH COMPLEX, CENTRAL IRANIAN
BLOCK
DISSERTATION
2009-09-09 Mahmoud Rahmati Ilkhchi
Institute of Petrology and Structural Geology
Faculty of Science
Charles University
Prague, Czech Republic
METAMORPHISM AND GEOTECTONIC POSITION OF
THE SHOTUR KUH COMPLEX, CENTRAL IRANIAN
BLOCK
PhD thesis by
Mahmoud Rahmati Ilkhchi
Supervised by
Prof. Ing. Shah Wali Faryad, CSc. and
Co-supervised by
RNDr. Petr Jerábek, PhD.
A thesis submitted to the
Faculty of Science, Charles University
of the requirements for the degree of
Doctor of Philosophy
Institute of Petrology and Structural Geology
2009
Jury Prague, 2009
Statement of originality
The results presented in this dissertation are outcome of my original scientific work in
collaboration with my supervisors and other colleagues. To the best of my knowledge
and belief, I declare that the results presented in my dissertation have not been
published or presented by someone else. This dissertation has not been submitted for
any other academic degrees at other university or educational institution.
Prague 2009
Mahmoud rahmati Ilkhchi
Statement of co-authors
On behalf of the co-authors, I declare that Mahmoud Rahmati Ilkhchi was the
principle investigator of the scientific study presented in this dissertation and
performed majority of the work. The contribution of the co-authors was mainly
supplying of the analytical data and critical comments on the final versions of the
manuscripts.
Prague, September 2009
Prof. Ing. Shah Wali Faryad, CSc.
Acknowledgements
I would like to thank all who supported me during my Ph.D. and without whom this
thesis would not be completed. So, thanks a lot …
I also would like to gratefully acknowledge my supervisor, Shah Wali Faryad, for his
scientific guidance as well as personal treatment during my struggle for getting to the
point, for many useful discussions, inspiration and big patience. Great thanks for his
help in the “transformism” combat.
I would like to extend my thanks to my co-supervisors Petr Jerábek for his scientific
guidance and for numerous valuable discussions as well as in field.
I would like to thank my colleagues, especially Holub for his scientific, personal
treatment and also great thanks for his help in analyzing of geochemistry of my work,
Koyi for his help in Structure Geology, Košler & Frank for age dating, Dolejš for
thermobarometry program, Tolar for his help in solving the problems, Schulmann,
Zavada, Lexa and Madams Wontrobová, Klápová
I also would like to extend my thanks to head of Geological Survey of Iran Mr.
Koraei and my dear colleagues in geological survey Ghassemi, Ghalamghash,
Farhadian, Eshraghi, and my colleagues in geological survey of Czech Republic
Venera, Mixa and Baburek.
And, at last but not least, I would like to thank my wife my children, father & mother,
who supported me and stayed by me during my Ph.D. course.
This work has greatly benefited from financial support of the Geological survey of Iran.
This work is dedicated to my wife and my sons.
Mahmoud
Publications:
1. Mahmoud Rahmati-Ilkhchi, Shah Wali Faryad, František V. Holub, Jan Košler, and
Wolfgang Frank. Magmatic and metamorphic evolution of the Shotur Kuh
Metamorphic Complex (Central Iran). International Journal of Earth Sciences (in
print)
2. Mahmoud Rahmati-Ilkhchi, Petr Jeřábek, Shah Wali Faryad and Hemin A. Koyi.
Jurassic–Pliocene tectono-metamorphic history of the Shotur Kuh metamorphic
complex in the Great Kavir Block (NE Iran). (in preparation)
3. Rahmati Ilkhchi M, 2002. Geological map of Razveh (scale 1:100,000 ).Geological
Survey of Iran, Tehran Iran
3. Zahedi, M., Rahmati Ilkhchi M, Vaezipour, J., 1994. Geological map of
Shahrekord (scale 1:250,000 ).Geological Survey of Iran, Tehran Iran
4. Rahmati Ilkhchi M, 2004. Geological map of Langerud (scale 1:100,000 ).
Geological Survey of Iran, Tehran Iran
5. Rahmati Ilkhchi M, Babakhani, A.R., Sahandi, M.R., Khan Nazer, N.H., 2005.
Geological map of Joghatay (scale 1:100,000 ).Geological Survey of Iran, Tehran
Iran
Conference abstracts:
1. Mahmoud Rahmati Ilkhchi, Shah Wali Faryad, Karel Schulmann, Jan Kosler.
2006. Metamorphism and exhumation processes of the Shotur Kuh Metamorphic
Complex, Semnan Province (Central Iran Zone). Geoline, 20, 55
2. Mahmoud Rahmati Ilkhchi, Shah Wali Faryad, Petr Jerabek, Jan Kosler. 2007.
Origin and Exhumation of the Shotur Kuh Core Complex. 5th Meeting of the
central European Tectonic Studies Group (CETeG),Tepla, Czech republic
3. Mahmoud Rahmati Ilkhchi, Shah Wali Faryad. 2008
Petrology and metamorphic evolution of the Shotur Kuh Complex in Central Iran.
Geosciences Conference, Tehran
4. Mahmoud Rahmati Ilkhchi, Petr Jeřábek, Shah Wali Faryad, Jan Košler. 2008.
Tectono-metamorphic evolution of the Shotur Kuh Metamorphic Core Complex in
the Central Iranian Block. 6th Meeting of the Central European Tectonic Studies
Group "CETeG", Upohlav, Slovakia
5. Mahmoud Rahmati Ilkhchi, Shah Wali Faryad, Jan Košler.
Metamorphism of the Shotur Kuh Complex, Central Iran. 2008.
86th
Annual Meeting of the Deutsche Mineralogische Gesellschaft, Berlin
6. Mahmoud Rahmati-Ilkhchi, Shah Wali Faryad, Peter Jeřábek, František V. Holub,
Jan Košler and Wolfgang Frank. 2009.
Magmatic and tectonometamorphic history of the Shotur Kuh Metamorphic
Complex, Rezveh area (Central Iran). 7th Meeting of the Central European
Tectonic Studies Group "CETeG", Pécs, Hungary.
7. Mahmoud Rahmati-Ilkhchi, Shah Wali Faryad, Peter Jeřábek, František V. Holub,
Jan Košler, and Wolfgang Frank. 2009.
Magmatism and metamorphic evolution of the Great Kavir Block (Central Iran).
Second Hindu Kush Conference, Kabul.
Table of Contents
INTRODUCTION 1
CHAPTER 1 2
Summary of Geology and metamorphism in Iran
1.1. Geology and tectonics of Iran
1.1.1. The Zagros Fold and Thrust Belt (southern unit) 3
1.1.1.1. The Zagros fold belt
1.1.1.2. The Zagros thrust zone 4
1.1.1.3. The Makran, Zabol–Baluch zone and the Eastern Iranian Ranges
1.1.2. The central units (Cimmerian block) 5
1.1.2.1. The Sanandaj Sirjan metamorphic zone
1.1.2.2. The Urumiyeh Dokhtar (Urumiyeh Bazman) magmatic arc
1.1.2.3. The Central Iranian zone 6
1.1.2.4. The Alborz zone
1.1.3. The northern unit 7
1.2. Distribution of basement rocks in Iran 8
1.2.1. The southern unit .
1.2.2. The central units (Cimmerian block) .
1.2.2.1. The Sanandaj Sirjan metamorphic zone 9
Region B 10
Region C 11
Region D 12
1.2.2.2. The Central Iranian zone 13
Region E
Region F 14
Region G 15
Region H 16
1.2.2.3. The Alborz zone 17
Region I
Region J
Region K
Region L 18
1.2.3. The northern unit
1.2.4. Summary of geochronological data on metamorphism of basement rocks in
Iran
1.3. References: 27
CHAPTER 2
Magmatic and metamorphic evolution of the Shotur Kuh Metamorphic Complex
(Central Iran) 37
Abstract
2.1. Introduction 38
2.2. Geological setting 39
2.3. Analytical method 41
2.4. Petrography 42
2.5. Geochemistry 44
2.6. Mineral chemistry 50
2.7. PT conditions of metamorphism 53
2.8. Geochronology 56
2.9. Discussion 59
2.10. Conclusions 65
2.11. References 67
CHAPTER 3 72
Mid-Cimmerian, Early and Late Alpine orogenic events in the Shotur Kuh
metamorphic complex, Great Kavir block, NE Iran
Abstract
3.1. Introduction 73
3.2. Geological setting 74
3.3. Structural record 79
3.3.1. Orthogneisses and amphibolites 80
3.3.1.1. Quartz deformation microstructures 81
3.3.2. Micaschists 83
3.3.3. The parauthochthonous cover sequences
3.4. Petrography and mineral composition 86
3.4.1. Orthogneisses and amphibolites 87
3.4.2. Micaschist
3.4.3. The parauthochthonous cover rocks 89
3.5. PT conditions of metamorphism
3.6. Discussion 93
3.6.1. Polymetamorphic record in Shotur Kuh complex
3.6.2. Tectonic evolution of the Shotur Kuh complex 95
3.6.3. Regional implications 98
3.7. Conclusions 100
3.8. References 101
CHAPTER 4 106
4.1. Summary and further research directions
4.2. References 109